Electrochemical Scanning Tunneling Microscopy: Adlayer Structure

J. Phys. Chem. C 2007, 111, 16109-16130

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FEATURE ARTICLE Electrochemical Scanning Tunneling Microscopy: Adlayer Structure and Reaction at Solid/liquid Interface Dong Wang† and Li-Jun Wan* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, China ReceiVed: May 15, 2007; In Final Form: July 6, 2007

The adlayer structure and reaction at the solid/liquid interface has been intensively investigated due to its importance in many physical chemistry and life processes. The emergence of electrochemical scanning tunneling microscopy (ECSTM) provides a new opportunity to study this subject at the atomic and molecular level in aqueous solution. This feature article summarizes some of the latest progress in ECSTM application for the understanding of the structure and reactions taking place at the solid/liquid interface. The adsorption of atoms and organic molecules is clearly revealed, and the knowledge of the underlying molecule/substrate and intermolecular reactions governing the adlayer formation on the surface is deduced. In addition, the effect of external stimuli, such as electrode potential and UV light, on the adlayer structure is demonstrated. Finally, the prospects of future development in this exciting field are outlined.

1. Introduction The solid/liquid interface is ubiquitous in nature and has been one of the most intensively studied subjects in physical chemistry. Many important processes take place at the solid/ liquid interface, such as wetting, adsorption, electronic and energetic transfer, and heterogeneous catalysis. The occurrence of electron and ion transfer reactions at the solid/liquid interface is the basis for technical applications of batteries, plating, and corrosion. Understanding the structure and reaction process at the solid/liquid interface is of great importance in surface science and electrochemistry and will benefit the study of catalysis, surface modification, sensors, biological processes, semiconductors, and electronic devices.1,2 Cyclic voltammetry is a typical traditional technique for the study of the solid/liquid interface. By precise measurement of the current and voltage, followed by sophisticated thermodynamic and kinetic treatment, rich information of the physical chemistry processes at the interface, such as diffusion, adsorption, electron and ion transfer, and reaction kinetics, can be obtained. On the other hand, the rapid development of surface science and analytical chemistry provides us with more surfacesensitive techniques for the study of the solid/liquid interface. For example, Fourier transformed infrared spectrometry,3-5 surface enhanced Raman scattering (SERS),6,7 quartz crystal microbalance (QCM),8 and ex situ high vacuum techniques9-11 have provided us tremendous information about the sold/liquid structure and chemical reaction with unprecedented preciseness. In 1982, Binnig and Rohrer invented scanning tunneling microscopy (STM).12 A great feature of STM is the ability to get real-time, real-space, three-dimensional surface structure * Corresponding author. E-mail: [email protected]. † Present address: National Institute for Nanotechnology, National Research Council, Edmonton, AB T6G 2M9, Canada.

information at up to atomic resolution in different environments such as ultrahigh vacuum (UHV), ambient, and even aqueous solution. The invention of STM has had a huge impact on surface science, and brought much exciting progress in surface and interface research. Not much later, electrochemical STM (ECSTM) was developed by adopting STM into electrochemical environment in solution.13-15 The setup with a bipotentialstat to independentantly control the potential of the electrode and STM tip is an important step in in situ ECSTM development and is widely used and commercially available now.16 ECSTM is a powerful tool with the ability to observe surface structures, monitor surface reactions, manipulate surface objects, and measure single molecular properties in aqueous solution. By means of ECSTM, and complemented with other spectrometry methods, significant advances have been made in the study of solid/liquid interface physical chemistry, such as the electric double layer structure, electrode surface reaction, surface nanostructure assembly, electrocatalysis, electrochemical single molecular devices, and interfacial biological electrochemistry. Some of these topics were covered by several comprehensive reviews previously.17-24 The previous studies have revealed the important roles of orientation and arrangement of surface adsorbates in physical chemistry such as enatioselectivity.25-28 On the other hand, with the development of nanoscience and nanotechnology, the study of the nature of surface adsorption and assembly will benefit the design and construction of new surface nanoarchitecture, which would be valuable for the fabrication of future nanodevices with higher integration degree and more effective performance. One key challenge for the application of the functional nano-objects is to find, assemble, and regulate them onto supported surfaces with defined structures.29 Sophisticated knowledge on the governing factors for the formation of an

10.1021/jp0737202 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007

16110 J. Phys. Chem. C, Vol. 111, No. 44, 2007

Wang and Wan

Dong Wang graduated from Nankai University in 1997 and obtained his Ph.D. in 2003 from the Institute of Chemistry, Chinese Academy of Sciences under the guidance of Prof. L.-J. Wan. From 2003-2006, he was a postdoctoral fellow and visiting fellow in the Department of Chemistry, Unversity of Alberta and National Institute for Nanotechnology (NINT), National Research Council (NRC), Canada. He is currently working as a research associate in NINT, NRC. His research interests are silicon surface chemistry and surface and interface selfassembly. Li-Jun Wan graduated from Dalian University of Technology of China in 1982 and earned a Ph. D. in materials chemistry in 1996 from Tohoku University of Japan. He joined the Institute of Chemistry, Chinese Academy of Sciences as a research professor in 1999 and is currently director general of the institute. His research has centered on physical chemistry, with an emphasis on molecular self-assembly, functional nanomaterials, electrochemistry, and scanning probe microscopy.

ordered surface adlayer is invaluable for the construction of surface nanostructures with controllable structure and function. Until now, UHV STM has already made a great contribution to the above-mentioned issues. However, by applying an electrode potential, it is possible to fabricate and tune the molecular adlayers or even organization of species on the surface, which is the special role of electrochemical STM concerning the above-mentioned issues.22 Therefore, in this feature article, we will focus on the ECSTM application in adlayer structures and reactions at the solid/liquid interface under potential control. Because the principle of ECSTM was previously reported in several review papers, we will briefly describe the EC-STM setup in this article. Although the electrode surface structure, the adlayer of specific adsorption of anion, and metal deposition on the electrode surface revealed by ECSTM in solution will be presented, the emphasis in the feature article is the adsorption and adlayer of organic molecules and supramolecules on the electrode surface from recent achievements. Owing to the length limitation of the article, we cannot list all of the research results in this field. However, the examples of surface electrochemical reactions and structure transformations will be shown. Furthermore, other interfacial reaction processes such as UV-lightinduced polymerization will also be introduced. The real-time observation ability of ECSTM allows us to monitor the adlayer structure change. Finally, the prospects of future development in this exciting field are outlined. 2. ECSTM Setup STM is operated based on the quantum tunneling effect. When bringing two biased electrodes close enough but not in contact yet, a weak current will pass through and be detected. The magnitude of this current is exponentially dependent on the distance between the probe and the surface. The phenomenon is called the tunneling effect and can be fully explained by quantum mechanics. Taking advantage of the tunneling effect, STM is designed with an ultrasharp tip as a probe and the biased conductive substrate as another electrode. By collecting the tunneling current between ramping probe and substrate, the surface structure information is reconstructed. STM can work in vacuum, ambient, and solution phase. ECSTM is an integrator of STM and four-electrode electrochemical bipotentialstat, as shown in Figure 1. The four-electrode setup is constructed in an electrochemical cell filled with aqueous solution. In this configuration, the potential of the STM tip (WE2 in Figure 1) and substrate (WE1 in Figure 1) can be independently controlled by a bipotentialstat, allowing the potential-dependent surface

Figure 1. Schematic diagram of ECSTM system.

structure and reaction to be monitored. In the real working condition, the current between tip and substrate is the combination of the tunneling current and Faraday current. To minimize the interference of the Faraday current, the ECSTM tip needs to be coated with a thin layer of insulator to minimize the electrode surface area of the tip and, thus, the associated Faraday current. The ultrasharp probe is typically made from electrochemically etched tungsten, Au wires, or mechanically cut PtIr wire. A thin layer of insulator, such as paraffin wax or nailpolish oil, is coated on the tip to minimize any possible leak of electrochemical current occurring, while the very tip of probe is exposed to pickup the tunneling current.30,31 A well-defined working electrode is another prerequisite for obtaining atomic level structural information of the electrochemical process. Clavilier et al. developed a flam-annealing method to prepare high-quality single-crystal noble electrode surfaces.32 The method works well for most of noble metals such as Au,33 Pt,32 Ir,34 Pd,35 Rh,36 etc. Besides flame-annealing, a well-defined electrode can also be prepared by vacuum sputtering and subsequent annealing. For the active metals like Cu,37 Ni,38 and Fe,39 the single-crystal electrode surfaces are always subjected to mechanical, electrochemical polishing, and potential annealing before use. Although it is undoubtedly accepted that the tunneling effect is the theoretical basis for STM operation, the precise interpretation of the atomic scale images has always been a concern and a problem. Generally speaking, STM images represent the electronic structure information of the surface near the Fermi level. The adsorbates interact with the surface and give a perturbation to the surface electronic structure, resulting in the change of tunneling current in their vicinity. Thus, the detailed substrate-adsorbates interaction can be obtained from a highresolution STM image. For example, Weiss and Eigler show that benzene molecules appear as three characteristic patterns in STM images depending on the adsorption sites on the Pt(111) surface.40 By using elastic scattering quantum chemistry approaches, Sautet and Bocquet were able to simulate the STM images, which agree pretty well with experimental results.41 So far, the basic understanding of the STM contrast mechanism has been achieved through the effort from the interplay between experimental and theoretical results. Detailed information can be found in the literature.42 3. Electrode Surface in Aqueous Electrolyte Solution Imaging the electrode surface is the first example to demonstrate the ability of ECSTM to study the solid/liquid interface. With atomic resolution, not only were the terrace and step on the single-crystal surface routinely seen but also the atoms on

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Figure 2. STM image (a) and structural model (b) of Au(111)-(22×x3) reconstruction. (Adapted with permission from ref 44. Copyright American Chemical Society.)

the surface. On a fresh, clean single-crystal electrode surface, the atoms in many cases retain their bulk arrangement, and thus a (1 × 1) surface structure is observed in STM images. Certain single-crystal electrode surfaces, however, may undergo reconstruction.43 The driving force for the formation of reconstruction is to release the strain associated with the asymmetry environment of surface atoms. The typical example is (22 × x3) reconstruction on Au(111) with the herringbone structure. Panels a and b in Figure 2 are an STM image and structural model, respectively, describing the special reconstructed structure.44 The reconstruction involves the compression of 23 toplayer atoms to fit into the space of 22 atoms in the bulk along the direction. At the electrochemical solid/liquid interface, surface reconstruction is potential dependent. Although the potentialdependent electrode surface structure change has been extensively investigated by traditional electrochemistry methods and other surface characterization techniques, ECSTM, for the first time, realizes the dream of electrochemists to visualize the electrode surface structure at the atomic level.43 Aside from reconstruction, this ability also makes ECSTM a great tool to study the surface dissolution,45,46 deposition,23,47 etching,48,49 and many other electrochemical processes. Now, an atomic level understanding of the electrochemical deposition and anisotropic etching has been achieved. Aside from the great achievement on these electrochemical processes, ECSTM is increasingly being recognized as a new tool for the study of alloy materials. As a next-generation energy source, fuel cells have attracted lots of interests from both fundamental and industrial perspectives. The high performance of fuel cells relies on new efficient catalytic electrode materials. For example, various Pt-based alloy electrodes were prepared, and their electrocatalytic properties were well investigated. A Pt-Fe alloy film exhibiting high CO tolerance toward H2 oxidation or O2 reduction has been reported.50 By using X-ray photoelectron spectroscopy (XPS), it is found that Fe atoms leached out into solution and a surface layer with a modified electronic structure formed on the film during the reaction. Figure 3 shows a cyclic voltammogram (CV) of such an electrode recorded in 0.1 M HClO4 solution. The dashed line is the first scan toward the positive direction from 0.5 V. The small peak at 0.71 V corresponds to the Fe2+/Fe3+ redox process. Previous electrochemical quartz crystal nanobalance and XPS results indicate Fe and Pt are dissolved into the solution, and Pt atoms redeposit on the electrode surface when it is in HClO4 solution. Repeated potential cyclings between 0.05 and 0.95 V lead to the decrease and finally disappearance of the small peak, indicating that potential cyclings can accelerate the Fe dissolution, help the rearrangement of Pt atoms, and form a protective Pt skin layer against further corrosion. After 10 scanning cycles, a steady CV similar to a polycrystalline Pt electrode is obtained

Figure 3. CV for a Pt-Fe film in 0.1 M HClO4 at a scan rate of 20 mV s-1. The solid line shows the steady-state CV, and the dashed line is the first scan in the positive direction from 0.5 V. The potential indicated by the arrow is ca. 0.71 V. (Reprinted with permission from ref 51. Copyright Royal Society of Chemistry.)

as shown by the solid line in Figure 3. To further reveal the relationship between the microstructure and the electrocatalytic activity of the electrode film, ECSTM was employed to study the surface structure of this new electrode material.51 Figure 4a is a typical STM image of a freshly made Pt-Fe film obtained in a N2 environment. The film is composed of well-crystallized grains in a size range from 50 to 100 nm. After the electrode is immersed into the electrochemical cell in 0.1 M HClO4 for 5 min, the surface morphology changed dramatically as shown in Figure 4b, indicating the occurrence of surface dissolution. Then, 10 potential scans between 0.05 and 0.95 V were applied on the alloy film electrode to induce the dissolution-redeposit process. Panels c-e in Figure 4 are sequential STM images of the surface morphology acquired at 0.5 V in 0.1 M HClO4 after potential scans. It is clear that the edges which were occasionally observed on the film surface, as indicated by arrows in Figure 4c, have become oriented crystalline steps in Figure 4d. After another 8 min (Figure 4e), the characteristic (111)-oriented facets with 3-fold symmetry and terrace-step structure are clearly resolved. Atomic resolution STM images obtained on the terrace show a hexagonal array of atoms. The atom packing direction is parallel to the edge direction. The interatomic distance is measured to be 0.28 ( 0.02 nm, which agrees well with the lattice constant of Pt(111). Therefore, it is concluded that a Pt(111)-(1 × 1) structure is obtained on the terrace. The ECSTM results clearly reveal the surface structure of the electrode surface. 4. Specific Ionic Adsorption When a biased electrode is brought into contact with an electrolyte, an electric double layer structure is formed. In this structure, ionic species adsorb on the electrode surface with opposite charge to form an inner shell, and the ionic species with the same charge as the electrode are loosely bound to form an outer shell.2 However, some weakly solvated anions, such as Cl-, Br-, I-, and SO42-, can lose part of the solvent shell


Wang and Wan

Figure 4. (A) Surface morphology of a fresh Pt-Fe film in nitrogen-saturated ambient conditions. (B) The surface of a Pt-Fe film immersed for 5 min in 0.1 M HClO4 at 0.5 V before the potential cycle. Images C-E show changes of the surface on Pt-Fe films at 0.5 V after 10 potential cycles. The time interval from image C to D is 40, and 8 min from image D to E. Image F is an atomic image acquired at the terrace area of image E. The scan area for images A-D is 300 × 300 nm2, for E it is 68 × 68 nm2, and for F it is 3 × 3 nm2. (Reprinted with permission from ref 51. Copyright Royal Society of Chemistry.)

and penetrate the double layer and strongly bind on the electrode surface. This adsorption process is called specific adsorption.52 Specific adsorption is a very important phenomena in electrochemistry and closely related to corrosion, plating, etching, and electrocatalysis.53-55 Using traditional electrochemical techniques, it is possible to obtain surface structure information such as coverage, adsorption status, and dynamic information. On the other hand, ex situ UHV surface characterization techniques, such as low-energy electron diffraction (LEED), electron energyloss spectroscopy, and Auger electron spectroscopy have also supplied important information on the structure of the specific adsorption.56,57 Generally, these techniques can provide average information on the surface. However, ECSTM will provide the opportunity to study in situ the specific adsorption at the atomic level in real space. High-resolution ECSTM images allow precise determination the local registry of atoms and ions on the surface. A detailed understanding of the phase formation and transformation of the adlayer structure with applied potential became possible. Halides are the most intensively studied anions that can form a specific adsorption adlayer on most metal electrodes such as Au,58 Pt,59 Ag,60 Cu,61 and Pd.62 Because of the strong interaction between metal and halides, the adlayer can exist stably even after the sample was taken out of the electrochemical cell, which benefits the application of ex situ UHV techniques to study the surface structure. Ordered adlayer structures of halides on metal electrodes depend on factors such as metalhalide interactions, electrode potential, and electrode crystallographic orientation. Both incommensurate and commensurate adlayer structures are observed by in situ ECSTM and correlated with the results by ex situ diffraction techniques. The formation of an iodine specific adsorption adlayer on Pt(111) was previously investigated by the combination of electrochemistry and ex situ LEED.63 The ordered adlayer structures depending on pH and electrode potential were determined. With the application of ECSTM, not only were the adlayer structures of iodine on Pt(111) observed in real space but the structural evolution with the applied potential was also revealed. Three different adlayer structures (x3×x3), (x7×x7), and (3 × 3) were successively observed with the increase of

the electrode potential.64,65 The adsorption models were proposed on the basis of the STM images obtained. The coverage for three adlayer structures is 1/3(0.33), 3/7(0.43), and 4/9(0.44), respectively. At 0 V, the coexistance of the (x7×x7) and (3 × 3) structures was observed. In situ STM observation shows that the potential-dependent phase transition is very slow, indicating that iodine has a strong interaction with the Pt surface.65 Iodine can also form an ordered adlayer structure on Au(111). However, the adsorption is relatively weak compared to Pt(111), resulting in the continuous changing of the unit cell with the applied potentials. Yamada et al. studied the iodine adlayer structure on the Au(111) surface with the combination of in situ ECSTM and ex situ LEED.66 Adsorption of iodine on Au(111) releases the (22×x3) reconstruction. At -0.07 V (vs Ag/AgCl), the highly ordered (x3×x3)R30° structure is observed. With the increase of the potential, the unit cell changes along the direction, whereas it keeps constant in the direction. To account for this electrocompress process, a general unit cell of c(p×x3 R-30°) adlattice is used. The value of p continuously decreases from 3 to 2.49 with increased electrode potential from -0.2 to +0.7 V (vs Ag/AgCl). At a higher potential region, a rotated hexagonal phase was observed by ECSTM. The moire´ pattern is shown in STM images for this rotated structure, which is formed by isometric compression of the iodine adlattice. A similar electrocompression process is observed for the adsorption of iodine on Ag(111).67 At a low potential, an ordered (x3×x3)R30° structure was observed. With the increase of the potential, the adlayer structure changed from c(p×x3 R-30°) to (x3r×x3r)R(30°+R), where 3 g p g 2.65, r < 1, 0 e R e 2.4° ( 0.5°. The surface coverage increases from 0.33 to 0.41 as well. Although not seen as often as halides or pseudohalide, sulfate also forms specific adsorption on some metal electrodes. In situ FTIR studies indicated that bisulfate/sulfate is the adsorbate on the electrode surface.68,69 From electrochemical measurements, it is known that sulfate/bisulfate forms an adlayer on Au(111) with coverage around 0.2.70 Magnussen et al. first reported the ECSTM study of sulfate/bisulfate on Au(111).71 Weaver’s group

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Figure 5. (a) CV of Rh(111) in 0.5 M H2SO4. (b and c) ECSTM images of sulfate adlayer on Rh(111) obtained at 0.5 V. (d) Proposed adlayer structure of sulfate/bisulfate on Rh(111) with coadsorbed water molecules. (Reprinted with permission from ref 73. Copyright American Chemical Society.)

investigated the same system by the combination of several different methods.72 They observed a (x3 × x7) structure. The unit cell is proposed to be composed of a sulfate/bisulfate ion and a hydrated water molecule, which gives a coverage of 0.2, in accordance with the electrochemical results. Wan and coworkers employed ECSTM to study the adlayer structure of sulfate on Rh(111).73 Figure 5a shows the CV of a flameannealed Rh(111) electrode in 0.5 M H2SO4. A sharp cathodic peak at ∼0.05 V corresponds to the displacement of adsorbed sulfate by hydrogen. Compared to the CV of the same electrode obtained in HClO4, the butterfly peaks at 0.64 V in 0.1 M HClO4 corresponding to the OH- adsorption/desorption disappear due to the adsorption of sulfate. STM was employed to reveal the structural detail of the sulfate/bisulfate adlayer. At 0.5 V, a highly ordered adlattice of sulfate/bisulfate covered the whole surface, as shown in Figure 5b. A point defect A and a dislocation B are clearly seen in the STM image. In the highresolution STM of a single domain shown in Figure 5c, the adlayer is composed of two lines of spots with different contrast, which are labeled as A and B in the figure. When it is compared with the atomic image of Rh(111), the unit cell is determined to be (x3 × x7), similar to the case of sulfate on Au(111). From high-resolution STM images, there is a weaker line of species alternatively inserted into the bright spots line. In the model proposed in Figure 5d, the bright spots in Figure 5c are ascribed to sulfate/bisulfate, so that the coverage of sulfate is 0.2, in accordance with the results by the radiochemical labeling method and electrochemistry.74 The oxygen atoms of sulfate are arranged on the atop position. On the other hand, the weak spots line can be ascribed to the water molecules coadsorbed on the electrode surface, since they are the only species other than sulfate in the solution. In the model, two layers of water molecules interconnected by hydrogen bonds are sitting between

the sulfate molecules line. A similar double layer water molecules adlayer structure on the metal surface has been obtained in UHV at low temperature.75 In the STM images, only the top layer of water molecules is seen. The (x3 × x7) structure is also seen in the adlyer of sulfate/bisulfate on Ir(111),34 Cu(111),76 Pd(111),77 and Pt(111)78 surfaces, indicating that the sulfate adlayer on these surfaces is the same in nature. A similar sulfate-water coadsorption model is proposed. At proper STM imaging conditions, both the top and bottom layers of water molecules are observed, indicating the accuracy of the model proposed in Figure 5d. The adsorption of sulfur atoms on the metal surface has received intensive investigation due to its important roles in catalysis and many electrochemical reactions. For example, S can poise a Cu-based catalysis reaction.79 In addition, adsorption and reaction of S with the Cu surface results in the formation of CuxS, which is technically an important semiconductor for solar cell application.80 Wang et al. studied the adsorption of sulfur atoms on Cu(111) in acidic conditions.81 The CV of Cu(111) recorded in 0.1 M HClO4 + 1 mM Na2S shows an anodic peak at about -0.15 V, which is attributed to the formation of a CuxS compound, and a cathodic peak shows at -0.27 V, which is related to the corresponding reduction. In situ ECSTM was employed to investigate the adlayer structure of sulfur on Cu(111) in the same solution. At -0.32 V, which is in the double layer potential region, a well-ordered S adlayer was clearly seen. Figure 6a is a typical STM image of a sulfur adlayer on the Cu(111) surface. Two domains were observed in the STM image. The higher resolution STM image shown in Figure 6b shows more details of the adlayer. The adsorbed sulfur atoms form a hexagonal array on the surface. The unit cell is determined to be (x7×x7)R19.1° by comparing it with the atomic image of Cu(111). This structure is also consistent with


Wang and Wan

Figure 6. (a) Large scale (20 × 20 nm2) and (b) high-resolution STM images (3 × 3 nm2) of the S adlayer on Cu(1 1 1) in 0.1 M HClO4 obtained at -0.32 V. (c) The corresponding structure model for (x7×x7)R19.1°-S adlayer. (d) STM image of the moire´ pattern obtained at -0.20 V. (Reprinted with permission from ref 81. Copyright Elsevier.)

the observation of rotational domains in Figure 6a. There are four bright spots at the corner positions and one spot with a different brightness at the centroid of one side of the unit cell. In the adlayer model proposed in Figure 6c, four sulfur atoms corresponding to the bright spots with high corrugation height in STM images are located at atop sites and form the frame of the unit cell, whereas two other S atoms in the cell are assigned to two threefold hollow sites which are related to fcc and hcp position, respectively. This model is similar to the (x7×x7)R19.1°-S on Pd(1 1 1) and (x7×x7)R19.1°-I on Pt(111).54,56 The so-constructed cell yields a coverage of 3/7 (≈0.43), consistent with the results obtained in UHV experiments.82 However, only one S atom in the cell was resolved in the STM image of Figure 6b. The corrugation height difference between the two S atoms in hollow sites can be explained as electronic effect due to the second layer of Cu atoms below the surface. The visible spot in STM image corresponds to the S atom in fcc position, and an invisible one in the hcp position. The (x7×x7)R19.1° adlayer structure is consistently resolved between the potential range from -0.35 to -0.22 V. Shifting the potential positively more than -0.22 V, a moire´ pattern is clearly seen, as shown in Figure 6d. The distance between the two centers of the moire´ pattern is measured to be ≈1.6 nm. The moire´ pattern in the STM images may be caused by the formation of an adlayer incommensurate with the substrate or adsorbate induced surface reconstruction. The adlayer structure of sulfur on Cu(111) is quite different in alkaline solution.83 The electrochemical cyclic voltammogram shows the formation of CuxS at -1.0 V, and its corresponding

reduction at -1.13 and -1.19 V. In situ ECSTM was used to investigate the potential dependent adlayer structures. At -1.3 to -1.05 V, a uniform (x3×x3) adlayer was observed on the surface. When setting the potential to slightly more positive than -1.05 V, an unusual triangular superstructure starts to evolve and finally extends over the whole surface, as shown in Figure 7a obtained at -1.0 V. The length of the sides of the triangular domain is ca. 3.8 nm, and each triangle contains about 50 sulfur atoms. Higher resolution STM images indicate that the sulfur atoms adopt a (x3×x3) structure inside the triangular domain. The triangular domains are separated by dark zigzag lines along the direction. A ring structure with the higher contrast than atoms inside the triangle is seen at the intersection of the dark lines. The unit cell is defined as (19 × 19). This (19 × 19) structure was observed at potentials between -1.05 and -0.98 V. The adlayer model is proposed in Figure 7b. Sulfur atoms sit at the 3-fold site of the Cu(111) substrate inside the triangular domain, forming a (x3×x3) structure. However, the sulfur atoms in the neighboring domains are located at hcp and fcc positions, respectively. These triangle domains are separated by the domain boundaries, which appear as dark zigzag lines in the STM images. At the intersection of domain boundaries, six sulfur atoms are arranged to form a hexagonal ring structure. These atoms are positioned at a 2-fold bridge site, which accounts for the higher contrast in STM images. At the potential beyond -0.98 V, copper sulfide starts to grow at the surface. A hexagonal adlayer with a unit cell of (x7×x7)R19.1° is observed by ECSTM. A structural model similar to that shown in Figure 6c is proposed for the S-terminated Cu2S(111)

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Figure 8. (a) Top view and (b) side view structural model for the Cu UPD adlayer on Au(111) electrode in sulfuric acid solution. The large gray, medium gray, small light gray, and filled spheres represent Au, Cu, S, and O atoms, respectively. (Reprinted with permission from ref 88. Copyright 1995 American Physical Society.)

Figure 7. (a) STM image of a (19 × 19) structure formed on Cu(1 1 1) in 0.1 M KOH+0.5 mM Na2S. Sample potential is -1.0 V. (b) Proposed structural model. A (19 × 19) unit cell is drawn by solid lines. (Reprinted with permission from ref 83a. Copyright 2003. The Electrochemical Society.)

monolayer. The distances between the nearest S-S ions on the S-terminated Cu2S(111) surfaces on Cu(111) is 0.391 nm, which is very close to that in the Cu2S crystal (0.393 nm). Because of the low mismatch, the eptaxial growth of Cu2S(111) layers up to 100 layers on Cu(111) was realized. 5. Metal Deposition The deposition of metal adsorbates on the electrode surface is among the most intensively studied electrochemical reactions and of both fundamental and technical importance.23,47,84 One interesting process in electrochemical metal deposition is underpotential deposition (UPD), which takes place at a potential positive to the Nernst potential of metal ions. The UPD process is typically observed when a foreign metal with a low work function is deposited on the electrode surface with a high work function. In situ ECSTM provides a useful tool to visualize the structural change on the surface at the atomic level.85 The UPD of Cu on Au(111) has been well studied by in situ ECSTM and other electrochemical surface characterization techniques.86,87 Electrochemical CV measurements of the Au(111) electrode in 1 mM CuSO4 + 0.05 M H2SO4 electrolyte solution reveals two reduction peaks at 0.2 and 0.05 V vs SCE, which correspond to UPD and bulk deposition of Cu, respectively. ECSTM and other surface characterization techniques have been empolyed to study the adlayer structures. Highly ordered adlayers with (x3×x3) and (1 × 1) structures were revealed by in situ ECSTM at the UPD potential and bulk deposition potential, respectively. However, the electrochemical measurement indicates that the coverage of the Cu UPD adlayer on

Au(111) is 2/3. A detailed analysis of STM data and the experimental results from the surface X-ray scattering technique88 indicate that the (x3×x3) structure in STM images actually corresponds to the adsorbed sulfate species on the Cu UPD adlayer. In the proposed structural model as shown in Figure 8, Cu adatoms organize into a honeycomb structure with the coverage of 2/3 and are sandwiched between Au(111) and sulfate anions, which sit at the center of the honeycomb cavity of the Cu UPD adlayer. It is believed that the strong interaction between Cu and sulfate stabilizes this unusual honeycomb UPD adlayer. The anions in the electrolyte solution play a big role in the UPD process. For example, the Cu UPD adlayer structures on Au(111) change significantly with the presence of ClO4-, Cl-, or other anions.89-91 The UPDs of various metallic ions on the electrode surface are still of great interests in electrochemistry and ECSTM. The effect of anions, organic additives, and the crystalline orientation of electrode surface on the adlayer structure have been intensively investigated.92-97 From the application respect, intensive attention is paid to the systems showing the potential catalytic properties and physical device applications. The latest advances in this topic can be found in several recent reviews.84 When setting the electrode potential negative to the Nernst potential of metal ions, bulk deposition takes place. Thin metal film depositions are generally classified as Fank-van der Merve, or layer-by-layer mode, and Stranski-Krastanov, or island growth mode. With the combination of ECSTM and some other surface experimental techniques, the dynamic process of electrochemical deposition is investigated. On the other hand, several new protocols to grow high quality films with interesting properties have been developed in recent years. The degree of mismatch between the lattice constant of foreign metal and the electrode surface is an important factor determining the growth mode of deposition process and the film quality. For example, Cu film grows with a Stranski-Krastanov mode on the Au(111) surface because of the large mismatch between the lattice constant of Cu and Au (12%). However, on the Au(100) and Ag(100) surface, a Cu film can grow up to 10 ML (on Au) or 8 ML (on Ag) with atomic flatness.98,99 ECSTM

16116 J. Phys. Chem. C, Vol. 111, No. 44, 2007 experiments indicate that the Cu film stacks up and forms a bcc(100) film on the Au(100) surface to minimize the lattice misfit. The results were further supported by in situ surface X-ray scattering (SXS) results.100 When the deposition of Cu on Au(100) goes beyond 11 ML, a striped feature appears in the STM images. High-resolution STM images reveal that the interatom distance in the ridge of the striped feature is about 0.26 nm, which is close to the lattice constant of Cu(111). In situ SXS measurements indicate the entire overlayer with nearly bcc film formed at lower coverages transformed into a microstructure composed of staggered, orthorhombic domains at 11 ML.101 Interestingly, this structural transition process involving the movement of a massive amount of surface atoms is completely reversible. The formation of atomic flat metal or semiconductor films has attracted lot of interest because of the great potential in physical devices applications. Although high quality epitaxy film can be obtained by UHV techniques, the electrochemical deposition has increasingly been recognized as a useful and efficient alternative. Sieradzki et al. reported the electrochemical defect-mediated growth (DMG) of metal films on the electrode surface.102 In this approach, the electrolyte is composed of the interested metal ions and less-noble mediator ions, which can be reversibly deposited on and striped off the electrode. The electrochemical experiment conditions were carefully tuned to periodically trigger the co-deposition of interested metal ions and mediator ions and to strip off the mediator ions. The defect sites created in each cycle served as the new nuclei site for the growth of the adlayer. In this way, a high density of clusters is maintained in the growing layer, which favors the layer-bylayer mode film deposition.103 Using this technique, the growth of Ag multilayers up to 250 ML mediated by Pb or Cu has been demonstrated. In situ STM is used to monitor the details of DMG process. A similar strategy has been applied to other system.104-107 Stickney’s group has developed electrochemical atomic layer epitaxy (EC-ALE) to grow high quality semiconductor films.47 As stated previously, the UPD process is a surface limited reaction and a monolayer of depostited metal can be formed at the proper condition. By alternatively depositing atomic layers of two compositions of the desired semiconductor, a high quality semiconductor film with a controllable thickness is obtained at room temperature using an electrochemical automated flow deposition system. The method has shown great versatility and successfully been applied to many technically important semiconductor films including CdTe,108 CdSe,109 GaAs,110 and others. 6. Adlayers of Organic Molecules Two-dimensional self-assembly of organic molecules is increasingly realized as a feasible bottom-up route for the fabrication of molecular-based devices in the future.111 So far, various sophisticated architectures have been achieved in ambient or UHV environments by using the noncovalent interactions, including van der Waals interactions, H-bonds, and π-π interactions.22,112-115 In contrast, adsorption of organic molecules at the electrochemical solid/liquid interface has its unique features. The electrode surface properties such as surface charge and thus molecule/substrate interactions can be tuned by the applied potential. The existence of different adsorbed species in the electric double layer can also involve in the formation of an adlayer on the electrode surface. From a fundamental point of view, the investigation on the organic molecules adsorption at the solid/liquid interface will be helpful in understanding the intermolecular interaction and molecule-

Wang and Wan TABLE 1: Adlayer Structures of Aromatic Molecules on Different Metal Electrode benzene naphthalene anthracene

Pt(111)

Rh(111)

Cu(111)

c(2x3 × 3) (x21 × x21)R10.9° disorder

c(2x3 × 3) (3 × 3) 3x3 × 3x3 disorder

(3 × 3) (4 × 4) (4 × 5)

surface interaction and how they determine the formation of self-assembled ordered structures. The information is of great value to guide the construction of self-assembly structures on the surface. On the other hand, the adsorption of organic adsorbates is closely related to the electrocatalysis, electrochemical reaction, photovoltaic applications, and many others. As a fundamental study of the formation of organic molecular adlayers on metal electrodes, the adsorption of small aromatic molecules such as benzene, naphthalene, and anthracene on different metal electrodes such as Pt(111), Rh(111), and Cu(111) is successively studied by ECSTM.37,116 The results are summarized in Table 1. Adsorbed benzene can form an ordered adlayer on Pt(111) with a symmetry of c(x3 × 3)rect-2C6H6 (coverage 0.17) in the double-layer charging region. Higher resolution STM images reveal a dumbbell feature for each adsorbed molecule, which is ascribed to the bridge-site adsorbed benzene molecules. At a negative potential, a new phase with a symmetry of (x21×x21)R10.9° was observed. The coverage for this adlayer structure is 0.14. Benzene forms the same c(x3 × 3)rect structure on Rh(111) as well, except with a larger domain size when compared with that on Pt(111). When sweeping the electrode potential close to the benzene desorption region, a new phase with (3 × 3) symmetry evolved. Each molecule appears as a triangular shape in higher resolution STM images, indicating benzene adsorbs at 3-fold site on Rh(111) in this structure. On Cu(111), benzene forms a highly ordered adlayer on the terrace.37 A (3 × 3) adlayer is seen in the whole double layer potential region. Each molecule appears as triangular spots, indicating it adopts 3-fold site of the substrate. The polycyclic aromatic molecules such as naphthalene and anthracene show different adsorption behavior than benzene on metal electrode.117 Although each naphthalene molecule adopting a flat-lying orientation is clearly resolved by high-resolution STM, no ordered adlayer is observed on Pt(111) because of the low mobility on the Pt(111) surface. In contrast, an ordered adlayer of naphthalene with a unit cell of (3x3 × 3x3) was observed on the Rh(111) surface. The naphthalene molecule shows a two-ring structure in high-resolution STM images with its C2 axis parallel to the [110] direction of the substrate. In the case of anthracene, no ordered adlayer is observed even on Rh(111), although it still adopts a flat-lying orientation on the surface. As a contrast, naphthalene and anthracene form ordered adlayer structures on the Cu(111) surface with the symmetry of (4 × 4) and (4 × 5), respectively.37 With the increase in aromatic rings, pyrene and perylene can only form short-range ordered structures on the Cu(111) electrode.118 The different behavior of aromatic molecules on the metal electrode qualitatively reflects the interaction between π electrons in aromatic molecules and the electrode surface. On Cu(111), the interaction is relatively weak and molecules are allowed to move around to form an ordered adlayer. Whereas on Pt(111), the interaction is so strong that the molecules are frozen once adsorbed on the surface. The substrate-molecule interaction on Rh(111) is somewhere between on Cu(111) and Pt(111), allowing the formation of an ordered adlayer of benzene and naphthalene. As demonstrated above, although the substrate-surface interaction is important for the adsorption process, the strong

Feature Article

Figure 9. High-resolution STM image (a) and model structure (b) of crystal violet adlayer formed on S-Au(111). The image was acquired in an area of 10 × 10 nm2 at 0.3 V. (Reprinted with permission from ref 120. Copyright American Chemical Society.)

interaction may potentially limit the mobility of molecules on the surface and prevent the formation of ordered adlayer. The molecule/substrate interaction, however, can be tuned by the insertion of an inert or active adlayer. For example, the adsorption of iodine or sulfur on the Au, Ag, and Pt surfaces is found to be able to act as a buffer layer for the formation of an ordered adlayer of functional organic molecules.119,120 As an example, Figure 9 shows the ordered adlayer of crystal violet molecular adlayer on sulfur modified Au(111) surface.120 First, ordered x3×x3 adlayer of S on Au(111) surface is obtained under electrode potential control.121,122 After the addition of crystal violet molecules into the electrolytes, the spontaneously self-assembled ordered structure as shown in Figure 9a is obtained. Each molecule appears as a characteristic triangular propeller shape, consistent with the molecular structure. The molecule adlayer shows a 3-fold symmetry. Similarly, porphyrin derivative 5,10,15,20-tetrakis(N-methylpyridinium4-yl)-21H,23H-porphine tetrakis(p-toluenesulfonate) (TMPyP) also forms an ordered adlayer on the sulfur-modified Au(111) surface. The molecules are close-packed into a 4-fold symmetry adlayer. The surface modification method described above is quite general. For example, the ordered adlayers of TMPyP on I-Au(111),123 I-Ag(111),124 I-Pt(100),125 and I-Pt(111)126 surfaces were successfully obtained. However, the detailed molecular arrangement in adlayers is more or less different depending on the substrate. In this regard, although the intermolecular interaction is dominated in the formation of ordered adlayers, the adatom/molecule interaction is definitely involved in the process as well. For example, the formation of an ordered TMPyP adlayer is relatively slow on S-Au(111) as compared with on the iodine-modified electrode. Other examples using the same strategy include the formation of adlayers of rhodamine on I/Au(111),127 fullerene derivatives on I/Au(111),128 and supramolecular squares on Cl/Cu(100).129


Figure 10. (a) Large-scale STM image of TCMB molecules on an Au(111) surface obtained at 0.63 V, I ) 1 nA. (b) Higher resolution STM image of TCMB molecules. (c) Structural model for the TCMB adlayer. (Reprinted with permission from ref 133. Copyright American Chemical Society.)

Kunitake et al. examined the surface morphologies of the fullerene adlayer on different electrode surfaces prepared by Langmuir-Blodgett transfer from the air-water interface.130 C60 molecules adsorb weakly on the iodine-modified Au(111) surface. Because of the high mobility, STM cannot visualize the C60 molecules. Highly ordered C60 arrays were obtained on bare Au(111) due to stronger and yet appropriate adsorption energy. Because of rapid rotational motion of C60 on Au(111), it appears as featureless single bright spots in STM images. On iodine-modified Pt(111) and bare Pt(111) surfaces, the adsorption energy is so strong that only randomly adsorbed molecular adlayers were observed. However, the internal details of fullerene become accessible by STM even at room temperature because strong substrate-molecule interactions freeze the perturbation motion of molecules. In addition, they also developed a new method to prepare a high quality fullerene adlayer from fullerene Langmuir-Blodgett films on iodine modified Au(111).131 The highly ordered fullerene adlayer on Au(111) is obtained by electrochemical desorption of the iodine adlayer after transfer of the fullerene onto I/Au(111). The adlayer quality is essentially the same as the epitaxial adlayer prepared by sublimation and much better than that prepared by direct transfer onto bare Au(111), in terms of low defects density and higher coverage. The intermolecular H-bonding, with the bond strength higher than most of the other noncovalent bonds, is an important intermolecular interaction in determining the self-assembly process. For example, a trimesic acid (TMA) molecule can form intermolecular H-bonging through three carboxylic acid groups which are symmetrical attached on a benzene ring and is a widely used ligand in crystal engineering. The adsorption and assembly of TMA and its derivatives on a Au(111) electrode surface has been investigated by ECSTM.132-134 One interesting hexagonal open honeycomb network adlayer stabilized by intermolecular H-bonding is observed. A similar network structure has been also obtained on HOPG and Cu in UHV and ambient environment.135-137 Figure 10 shows the STM images

16118 J. Phys. Chem. C, Vol. 111, No. 44, 2007 of the adlayer of a TMA derivative, 1,3,5-tris(carboxymethoxy)benzene (TCMB), on Au(111) to illustrate this H-bonding stabilized structure. From a large-scale STM image (Figure 10a) acquired at 0.63 V, it can be clearly seen that TCMB molecules are self-organized into 2D honeycomb networks with a cavity in the center. In the higher resolution STM image shown in Figure 10b, more details of the arrangement of individual TCMB molecules in the honeycomb networks are revealed. Each TCMB molecule is resolved as a propeller shape, consistent with the chemical structure of the molecule. This feature suggests that TCMB molecules adopt a flat-lying geometry on Au(111). Apparently, the three blades of each propeller should be attributed to the three carboxymethoxyl groups of TCMB molecules. The adjacent TCMB molecules around a cavity have alternative orientations, as marked in Figure 10b. From a comparison with the orientation of the underlying Au(111) lattice, a (6 × 6) structure for the molecular adlayer can be concluded. A unit cell is superimposed in Figure 10b. Each unit cell includes two molecules. An illustrative structural model is proposed in Figure 10c. The honeycomb network is stabilized by the H-bonding between adjacent TCMB molecules. It is noted that, unlike the head-to-head arrangement of molecules in TMA network, TCMB molecules adopt an interdigitated arrangement to accommodate the bigger size of TCMB molecules while keeping good coordination with the substrate. As a comparison, 1,3,5-tris(3-carboxypropoxy)benzene (TCPB) cannot assemble into the honeycomb network structure because the flexible alkyl chains in TCPB cannot support the formation of a low coverage open network structure.133 Klymchenko and co-workers investigated the assembly of 5-hexadecyloxy isophthalic acid (ISA 16) on a Au(111) surface by ECSTM.138 ISA 16 has an aromatic head group with two carboxylic acid groups and a flexible long alkyl tail. The ordered lamella adlayer is obtained at the proper electrode potential. STM images reveal that ISA 16 molecules are arranged in a head-head configuration in the lamella structure. The hydrophilic isophthalic acid groups interact with each other through intermolecular hydrogen bonds, whereas the hydrophobic alkyl tails are packed interdigitatedly. The lamella run along the NNN (next-nearest neighbor or ) direction of Au(111). The adlayer is formed as a result of the commensurability of the ISA aromatic residues with the gold substrate and the intermolecular H-bonding of the ISA residues. On the other hand, the absence of specific alignment of alkyl chains related to the Au(111) surface indicates incommensurability of alkyl chains on Au(111), compared to on HOPG. van der Waals interaction between alkyl chains and the HOPG surface is one of the most important interactions to construct ordered self-assembly on the HOPG surface.114 On the Au(111) surface, it is found that reconstruction is the key factor to obtain an ordered alkane adlayer, because the alkane monolayer geometrically matches well with the reconstructed surface.139,140 One interesting feature of the alkane monolayer on the reconstructed Au(111) surface is the molecular rows of evenand odd-numbered alkanes ran in the nearest-neighbor (NN) atomic direction and the NNN atomic direction of the gold surface, respectively. In the electrochemical environment, hexadecane is found to self-assemble into ordered layers over the potential range from 0.15 to 0.55 V (vs SCE), on both reconstructed and unreconstructed Au(111) surfaces.141 The potential dependent phase transition is studied by in situ ECSTM. The metallamacrocyclic supramolecular assemblies represent a class of interesting materials not only bearing a beautiful

Wang and Wan topological structure but also possessing magnetic, photophysical, electronic, and redox properties that may not be accessible from purely organic systems.142-145 Fabricating desirable and stable devices from these assemblies on solid surfaces and understanding the rules governing their self-organization on solid supports are significant fundamental steps toward realizing useful nanodevices and nanostructures. Previously, several supramolecular assemblies bearing interesting properties have been deposited on a surface and examined by STM.146-149 For example, the self-organization of [2 × 2] grid-type Zn(II) and Co(II) complexes on highly oriented pyrolytic graphite (HOPG) has been investigated and shows the potential as high-density storage devices.150,151 Yuan et al. studied the adsorption and self-organization of several different supramolecular metallamacrocyclic assemblies on the Au(111) electrode.152,153 For example, the supramolecular rectangle [(1,8-bis(trans-Pt(PEt3)2)anthracene)(4,4′-bpy)]2(PF6)4 is found to form a highly ordered molecular adlayer with a domain size larger than 100 nm × 100 nm, as shown in Figure 11a. The underlying Au(111) reconstruction has no effect on the formation of the ordered adlayer. The adlayer has a rectangular unit cell of (7 × 3x3) structure. As revealed by high-resolution STM images (Figure 11b), each molecule consists of a set of four bright spots, which correspond to the aromatic rings of the supramolecular rectangle, with dimensions of 2.0 nm × 1.2 nm, consistent with the size of the rectangle determined from single-crystal X-ray crystallography.154 The supramolecular rectangle adopts a flat-lying configuration on the Au(111) surface, and a dark depression is seen in the center of each rectangle. Similar to the supramolecular rectangle, two other supramolecular assemblies with square and cage shapes are found to form an ordered adlayer on Au(111) as well.152 The internal structure details and the orientation of supramolecular assemblies to the surface are determined by the high-resolution STM images. However, these supramolecular assemblies cannot form an ordered stable adlayer on the HOPG surface, which is mainly due to the weak interaction between molecules and the HOPG surface. When mixing the rectangle and cage supramolecules together, they tend to separate to form individual domains on the surface. 7. Multicomponent Adlayers Building up complicated assemblies is one big challenge for the future of bottom-up nanodevice fabrication. Multiple component self-organization processes on the surface to form complicated and stable adlayers are receiving increased attention due to the possibility to integrate molecules with different functions and to fabricate novel nanostructures. Through selforganization, multiple components could be arranged on a surface with precise geometry and function, which is mainly dominated by the intermolecular interaction such as van der Waals forces,155 H-bonding,156 π-π interaction,157 and metalligand interactions.158 Fullerene attracts much attention because of its unique physical and chemical properties and myriad potential applications.159,160 Construction of an ordered array of fullerene is very important for device fabrication based on fullerene. However, the C60 molecular adlayer on Au(111) appears to be highly mobile, and C60 molecules quickly diffuse on the surface.161,162 On the other hand, it is known that fullerene can form a stable host-guest complex with host molecules such as calixarenes,163 which are found to form ordered adstructures on Au(111). By taking advantage of the host-guest interaction between calixarene and fullerene, an ordered fullerene array is obtained on Au(111).

Feature Article


Figure 11. (A) Large-scale STM image (E ) 550 mV, Etip ) 392 mV, Itip ) 941.0 pA) of the self-assembled supramolecular rectangles adsorbed on a Au(111) surface. (B) High-resolution STM image (E ) 550 mV, Etip ) 370 mV, Itip ) 765.0 pA) of the rectangular adlayer. Underlying Au(111)-(1 × 1) lattice is shown inset. (C) Proposed structural model for the adlayer. (Reprinted with permission from ref 152. Copyright American Chemical Society.)

Self-assembly of different calixarene molecules has been studied by STM and AFM.164-166 We have carried out an ECSTM study of self-organization of a serials of calix[4]arene, calyx[6]arene, and calyx[8]arene bearing different functional groups on Au(111).167 The calixarene molecules form ordered self-organizations on the Au(111) surface, whose orientation and conformation are dependent on the chemical structure such as the low rim groups of the calixarenes. Here, we describe the adlayer of a calix[8]arene derivative (C104H128O24: OBOCMC8) in detail.168 Then, a C60/OBOCMC8 inclusion complex is synthesized, and a stable and ordered C60/OBOCMC8 complex adlayer is prepared on Au(111). The detail information of molecular orientation and conformation of OBOCMC8 and C60/ OBOCMC8 in the self-organized molecular adlayers were investigated by STM. Figure 12a is a typical STM image of the OBOCMC8 adlayer on the Au(111) surface in 0.1 M HClO4. A highly ordered adlayer of OBOCMC8 extends over the atomically flat terrace of the Au(111) surface. The structural details are revealed by a higher resolution STM image (Figure 12b). The molecular adlayer consists of regular rows of round shaped calix-like features with dark depressions in the center. These features are easier to recognize in the height-shaded surface plot shown in Figure 12c. The distance between the centers of the dark depressions is 1.2 ( 0.1 nm along the A direction (equivalent to the direction; Figure 12b) and 1.7 ( 0.1 nm along the B direction. The molecular rows in the A and B directions cross each other at an angle of 95 ( 2°. A unit cell for the OBOCMC8 adlayer is defined in Figure 12b. The dimension of unit cell matches reasonably with the size of the OBOCMC8 molecule and, therefore, each calix-like feature appearing in the STM image can be assigned as an individual

OBOCMC8 molecule. In the structural model proposed in Figure 12d, each molecule is adsorbed on the surface in an upright configuration through the carboxyl-gold interaction. The surrounding protrusions and the dark depressions of calices in the STM images are attributed to the phenyl groups and the molecular cavity, respectively. A self-organized array of the C60/OBOCMC8 complex on a Au(111) surface was also investigated by ECSTM, and a typical image is shown in Figure 13a. An interesting feature of the C60/ OBOCMC8 adlayer is the well-ordered bright spots, which appear to fill the dark depressions that were observed in the OBOCMC8 adlayer, presumably due to the inclusion of C60 molecules into the OBOCMC8 hosts. In the higher resolution STM image shown in Figure 13b, a two-dimensional hexagonal adlattice of the C60/OBOCMC8 was revealed. The molecular rows are parallel to the direction of the Au(111) substrate and cross each other forming alternating 60° or 120° angles with an experimental error of (2°. The interdistance of bright spots along the molecular rows is measured to be 1.4 ( 0.1 nm, which is approximately the size of the C60/OBOCMC8 complex. Careful examination shows that the individual bright spots, indicated by the arrow in Figure 13b, are surrounded by circular protrusions. As shown previously, the C60/OBOCMC8 complex tends to be in the upright configuration on the Au(111) surface bonded through the carboxyl groups. Therefore, the bright spot and circular protrusion can be assigned as C60 and phenyl units, respectively, with each OBOCMC8 cavity filled with a C60 molecule. These structural details can be more clearly seen in the height-shaded surface plot (Figure 13c), and a structural model for the C60/OBOCMC8 adlayer is proposed in


Wang and Wan

Figure 12. (a) Large scale STM image of OBOCMC8 adlayer on Au(111). Top view (b) and height-shaded surface plot (c) of a high-resolution STM image of the adlayer. (d) Proposed structural model. The biased voltage and tunneling current were -213 mV and 670 pA, respectively. (Reprinted with permission from ref 168. Copyright Wiley.)

Figure 13. (a) Typical large scale STM image of C60/OBOCMC8 adlayer on Au(111). (b) Top view and (c) height-shaded surface plot of a high-resolution STM image of the adlayer. (d) Proposed structural model. The biased voltage and tunneling current were -140 mV and 1.0 nA, respectively. (Reprinted with permission from ref 168. Copyright Wiley.)

Figure 13d. The unit cell for the adlayer is outlined in Figure 13, panels b and d. On the basis of the STM observations, it is clear that both the OBOCMC8 and C60/OBOCMC8 complexes adsorb on

Au(111) surfaces and self-organize into well-ordered adlayers. They both adopt an upright configuration on the Au(111) surface, which is dominated by the carboxyl/Au interaction. The cavity of the OBOCMC8 molecule appears as dark depressions,

Feature Article

Figure 14. (a) Composite STM image (15 × 15 nm2), acquired at 0.85 V versus RHE, of a layer of the C60 cage/[Zn(oep)] supramolecular assembly on Au(111) in 0.05 M H2SO4. The potential of the tip was 0.35 V. Tunneling currents were 0.03 nA (upper part) and 2.0 nA (lower part), respectively. (b) Structural model of the supramolecularly assembled C60 cage/[Zn(oep)] layer on Au(111). (Reprinted with permission from ref 176. Copyright Wiley.)

which are filled with C60 molecules in the C60/OBOCMC8 complex. The same strategy could be used to assemble other fullerene derivatives, clusters, and organic molecules. The soobtained architecture with stable configuration and well-defined array should be interested in nanodevice and sensor applications. The porphyrin-fullerene system is increasingly recognized as a potential candidate for photovoltaic device because of its superior properties in photoinduced energy- and electron-transfer processes.169,170 Recent studies have shown that fullerene and porphyrin can form supramolecular host-guest complexes in cocrystallite state due to the strong π-π interaction between them.171-173 On the other hand, two-dimensional self-assembly of the porphyrin-fullerene complex on the metal surface is relatively less studied.174,175 Yoshimoto et al.176 studied the interface self-assembly of porphyrin and an open cage C60 derivative on the Au(111) surface. Detailed ECSTM observation reveals the orientation and arrangement of the porphyrinfullerene complex on the surface. The self-assembly process was completed by sequentially dipping the Au(111) surface into a benzene solution of zinc(II) octaethylporphyrin Zn(OEP) and the open C60 cage. A highly ordered array of the C60 cage on the [Zn(OEP)]-modified Au(111) adlayer was seen in the STM image. Each complex appears as a bright spot in the STM images. In contrast, a disordered structure was found when the C60 cage molecules were directly attached to the Au(111) surface. Higher resolution STM images were able to reveal details on the internal structure, orientation, and packing arrangement of the supramolecular assembled layers of the C60 cage/[Zn(OEP)] on Au(111). Figure 14a is the composite STM image of the C60 cage adlayer on the Zn(OEP) modified Au(111) surface. The upper part is recorded at low tunneling current

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16121 conditions, and each open C60 cage is seen as a bright spot. When the STM imaging condition is changed (lower part in Figure 14a), the underlying Zn(OEP) adlayer is seen in the STM images, providing further evidence of the formation of 1:1 supramolecular assembly. Each [Zn(OEP)] molecule can be recognized as a square with eight additional spots at the corners, which correspond to the eight ethyl groups. Similar to the adlayer of [Co(OEP)] and [Fe(OEP)] on Au(111),177 the Zn(OEP) adlayer consists of two alternated rows of Zn(OEP) molecules with different orientations. A structural model consistent with the STM observation is proposed in Figure 14b. Because of the specific interaction of Zn(OEP) and C60 cage, all of the C60 cage molecules have the same orientation in the adlayer, which is confirmed by electrochemical measurement. A similar ordered assembly of the C60 cage is also observed on the Zn(OEP) modified Au(100)-hex surface.178 Interestingly, aggregates of opened C60 were found on the ordered Zn(OEP)modified Au(100)-(1 × 1) surface. As a result, poor electrochemical response of the C60 cage is found on Zn(OEP)modified Au(100)-(1 × 1). The difference in surface charge of the unreconstructed Au(100)-(1 × 1) and the reconstructed Au(100)-(hex) surface is thought to affect the molecular recognition between the opened C60 cage and Zn(OEP). The supramolecular assembly of the ferrocene-linked C60 derivative (C60Fc) with other metal ions coordinated octaethylporphyrin (MOEP) was also investigated by ECSTM.179 Highly ordered C60Fc arrays on the Zn(OEP)-, Co(OEP)-, and Cu(OEP)modified Au(111) surface were observed by the formation of the 1:1 porphyrin-C60Fc complex. The well-defined electrochemical response of the Fc group in C60Fc was obtained due to the control of the orientation of C60Fc molecules. In contrast, a disordered structure of C60Fc was found on the FeCl(OEP)modified Au(111) surface because the presence of the Cl ligand prevents the formation of a supramolecular assembly. Accordingly, an ill-defined unclear electrochemical response of the Fc group is observed. Porphyrin and phthalocyanine have similar structures and are found to be the core building elements in many important functional proteins.180 Because of their superior electronic transfer ability, they have great potential to be used in photovoltaic and molecular devices and so on. Constructing ordered adlayers of porphyrin, phthalocyanine, or both at metal surfaces has attracted a lot of interest.181,182 Suto et al. investigated the binary assembly of cobalt(II) phthalocyanine (CoPc) and copper(II) tetraphenyl-21H,23H-porphine (CuTPP) on the reconstructed Au(100)-(hex) surface.183 The adlayers were prepared by immersing the crystal in the benzene solution of organic molecules and observed by ECSTM in 0.1 M HClO4. Higher resolution STM images revealed that CuTPP preferably adsorbed on the reconstructed Au(100)-hex surface, and the adsorption CoPc, in contrast, lifts up the reconstruction and forms an ordered adlayer on Au(100)-(1 × 1). The composition and structure of the mixed adlayer consisting of CoPc and CuTPP molecules varied with immersion time. CoPc molecules displace CuTPP molecules with increasing immersion time, accompanied by the lift up of the Au(100)-hex reconstruction. An ordered supramolecular adlayer with alternate CoPc and CuTPP molecular rows on Au(100)-(hex) is formed at the immersion time of 5 min. The potential-induced order/disorder transformation is found at the potential positive rather than the open circuit potential, due to the release of the underlying Au(100)-hex reconstruction. A similar immersing-time dependent structure change was found in the mixed adlayers of CoPc and copper(II) octaethylporphine (CuOEP) on Au(111).183 CoPc


Wang and Wan

Figure 15. (a) Large scale and (b) high-resolution STM images of 22BPY molecules on Cu(111) at -0.25 V vs RHE. (c) Cross-sectional profile of 22BPY adlayer along S-S′ in (b). (d) A high-resolution STM image of the 22BPY adlayer obtained at 0 V. (Reprinted with permission from ref 192. Copyright Wiley.)

molecules displaced CuOEP molecules during the modification process with the increase of immersion time. The ordered twocomponent adlayer was found to form either a p(9 × 3x7R 40.9°) or a p(9 × 3x7R - 19.1°) structure. The adlayer consists of two alternate molecular rows of CoPc and CuOEP with the orientation slightly different that in the adlayer on Au(100)hex. The surface mobility and the molecular reorganization of CuOEP and CoPc were accelerated by modulation of the electrode potential. The ordered binary domains disappeared, and the two components tend to form phase separation domains upon potential modulation. 8. Phase Transformation with Electrode Potential One intriguing feature of the electrochemical solid/liquid interface is that the electrode surface charge density can be modulated by the electrode potential. In response to the accompanying molecule/substrate interaction and/or intermolecular interaction change, the adlayer may go through phase transformation or adsorption orientation transformation.184 The phase behavior of the adlayer not only provides us the information of underlying interactions in determining the adstructure but also gives us a feasible route to obtain the desirable ordered two-dimensional nanostructure. The self-assembly of 5,10,15,20-tetra(4-pyridyl)-21H,23Hporphine (TPyP) on Au(111) electrodes was investigated by in situ ECSTM.185 It is found that the mobility of molecules on the surface is strongly dependent on the electrode potential. At positive potentials (>0.5 V vs SCE), a disordered adlayer of TPyP is formed on the Au(111) electrode, and the disordered molecules are immobile. At negative potentials (-0.2 V), however, the molecules are highly mobile and difficult to be imaged by STM. A highly ordered adlayer is formed at intermediate potentials (-0.2 to +0.2 V). Once the ordered adlayer is formed, it persists even after the potential is stepped to higher values (0.5-0.8 V). Potential modulated adsorbate-

substrate interactions and surface mobility is proposed to explain the observed results. The interplay of the electrochemical redox reaction and the adsorption status is also investigated.186 The molecules with N-containing heterocycles show strong interaction with metal surfaces. The phase transformation of uracil and cytosine on Ag, Au, and Hg electrode surfaces were studied using surface enhanced infrared adsorption spectroscopy (SEIRAS), electrochemistry methods, and in situ STM.187,188 2,2′-Bipyridine (22BPY) and 4,4′-bipyridine (44BPY) show diverse phase behavior on metal electrode surfaces and have been intensively studied as a model system to understand the potential dependent phase transition.189,190 Tao and co-workers studied the structure evolution of 22BPY monolayers on the Au(111) substrate as a function of the substrate potential.191 22BPY molecules adsorb vertically onto the substrate through two nitrogen atoms facing the Au(111) surface. At high potential, the vertically standing 22BPY molecules stack into polymeric chains like rolls of coins. Two adjacent molecules in a molecular row offset each other at ∼1.5 Å to avoid the perfect alignment of π-electrons in pyridine rings. At low potential, the individual chains are randomly oriented. The reversible order-disorder transition happens at a critical potential, which is partially dependent on the coverage of 22BPY on the surface. The phase transition is believed to be driven by a potential dependent attractive force between the chains. The adsorbed 22BPY molecule perturbs its surrounding local surface potential and, thus, modulates the local adsorption energy. When the local adsorption energy becomes larger than the thermal energy, a disorder/order phase transition will occur. This hypothesis is supported by a theoretical investigation of the local surface potential using a self-consistent density functional method. The adsorption orientation transformation of 22BPY on a Cu(111) surface in HClO4 solution was studied by using ECSTM and SEIRS.192,193 Figure 15a shows a typical STM

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Figure 16. SEIRAS spectra of 22BPY molecules on Cu(111) with electrode potentials from -0.25 to 0.1 V at a scan rate of 5 mV s-1. (Reprinted with permission from ref 192. Copyright Wiley.)

image of a 22BPY adlayer at -0.25 V. The adlayer consists of paired rows. Two spots in a paired row form an ellipse, and each ellipse indicated in the figure represents an individual 22BPY molecule. A higher resolution STM image (Figure 15b) allows us to ascertain the structural details. Each 22BPY molecule appears in a dumbbell shape with two blobs. The distance between the centers of two adjacent blobs is ca. 0.4 nm, as expected from the chemical structure of a 22BPY molecule. Thus, the two blobs are attributed to two pyridine rings of a single 22BPY molecule. A careful observation indicates that a height difference exists in a 22BPY molecule between two pyridine blobs. The corrugation height difference is measured to be ca. 0.02 nm along S-S′ as shown in Figure 15c, which can be attributed to a molecular torsion from trans to cis transition. The molecular distance in the same rows along is ca. 0.42 nm. The theoretical width of a 22BPY molecule from the N atom to the opposite H atom is ca. 0.4 nm. From the STM image and the chemical structure, it was determined that the 22BPY molecule assumes a flat-lying orientation on the Cu(111) surface. It was clearly seen that the molecular orientation varied when a more positive electrode potential was applied. Figure 15d is a higher resolution STM image recorded at 0 V, which shows different molecular features than the images recorded at -0.25 V vs RHE (reversible hydrogen electrode). The “thickness” of each molecule is measured at ca. 0.35 nm. From this it is reasonable to consider that at 0 V vs RHE, the 22BPY molecules reside vertically on the Cu(111) surface (Figure 15d). The orientation transition from flat-lying to vertical is completely potential dependent and reversible. SEIRAS results confirm the orientation of the 22BPY molecule on the surface as shown in Figure 16. Because of the selection rules of surface infrared spectra, no signal was detected for the in-plane vibration as observed at -0.25 V when the molecule assumes a flat-lying orientation on the electrode surface. As the electrode potential is scanned to positive values, peaks emerge with increasing intensities at 1597 and 1486 cm-1, which are assigned to the adsorbed 22BPY molecules. At 0.1 V, the intensity reaches the maximal value and remains

Figure 17. (a) Large-scale STM top view of 44BPY on Cu(111). The images were obtained at -0.20 V with a tunneling current of 8.6 nA. (b) High-resolution STM image from (a). The inset in (b) shows an image of the Cu(111) (2.2 nm × 2.2 nm) observed before injecting 44BPY. (c) The HOMO and LUMO of 44BPYH22+. (Reprinted with permission from ref 194. Copyright American Chemical Society.)

consistent until 0.3 V, indicating that the molecule rearranges and assumes a vertical orientation, confirming that the orientation of the 22BPY molecules on the Cu(111) surface can be tuned by adjusting the electrode potential. The adsorption of 4,4′-bipyridine (44BPy) on Cu(111) was also investigated in HClO4.194 44BPy is protonated in acid media and exists in the form of 44BPYH22+. Figure 17a shows the typical STM image obtained at -0.2 V. A well-defined molecular array is seen to extend over the atomically flat terrace. A higher resolution STM image (Figure 17b) reveals that 44BPYH22+ molecules adsorb in a flat orientation on the Cu(111) surface and form a well-ordered monolayer with a (3 × 4) symmetry. The observed STM image is similar to the HOMO of BiPyH22+ (Figure 17c). When changing the electrode potential to a more negative region, a new structure emerged and finally covered the whole surface after several minutes. Figure 18a is a typical large-scale STM image acquired at ca.


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Figure 18. (a) Large-scale STM image of 44BPY adlayer on Cu(111). Imaging conditions are E ) -0.36 V and tunneling current 8.2 nA. (b) High-resolution STM top view of 44BPY adlayer on Cu(111). (c) Cross-sectional profile along the A-B direction in (b) showing the corrugation height difference in a molecular row. (Reprinted with permission from ref 194. Copyright American Chemical Society.)

-0.36 V. Molecular domains marked as A and B are observed on the same terrace. The two domains cross each other at an angle of ca. 150°. The higher resolution STM image in Figure 18b reveals that the adlayer is composed of the alternative rows of types I and II. The molecules in row I are aligned along the direction of the underlying Cu(111) lattice, whereas those in row II are along the direction. The angle β between the alignment directions of molecules in two neighboring rows is 150° ( 2°. The size of each molecule is ca. 0.68 nm in length, which is close to the length of 44BPYH22+ (ca. 0.7 nm). The intermolecular distance in the same row is measured to be ca. 0.4 nm, close to the typical stacking distance of heterocyclic aromatic molecules. The results suggest that 44BPY molecules adopt a perpendicular orientation at this potential. More interestingly, it is found that the molecules have alternatively different brightness in each row. As shown in the cross-sectional profile (Figure 18c), the corrugation height difference of two neighboring molecules along the molecular row is measured to be ca. 0.03 nm. To supply further information on the surface adsorption state of 44BPY on Cu(111), a SEIRAS study was carried out. A detailed comparison with literature data and DFT simulation results indicates that the SEIRA spectrum is largely different from the normal IR spectrum of 44BPYH22+ in the solid state and rather resembles the normal Raman spectrum of 44BPYH22+ in acid and the SERS spectrum of 44BPYH22+ adsorbed on an Ag electrode.195 The band at 1643 cm-1 indicates protonation of 44BPY in acid electrolyte. At E < -0.3 V, four new bands emerge at 1597, 1501, 1336, and 1001 cm-1. The spectral features are almost identical to those for the self-dimers of (or one-dimensionally stacked) monocation radicals of methyl- and heptyl-viologens (N,N′-dialkyl-4,4′-bipyridium cations).196,197 The radicals interact with each other with a face-to-face configuration in the self-dimers. This result strongly suggests that 44BPYH22+ is reduced to the monocation radical, 44BPYH2+•, and forms a face-to-face self-dimer (or onedimensional stack) on the electrode surface at E < -0.3 V.

The SEIRA spectra of both 44BPYH22+ and 44BPYH2+• are dominated by gerade modes which should be IR-inactive for the centrosymmetric species. The breakdown of the selection rule of IR absorption is ascribed to the vibronic coupling associated with charge transfer between 44BPYH22+ and the surface and between the radicals. The dimer structure is also in accordance with the molecular pairs in the STM images. Similar electron-transfer induced reorientation is also observed in the benzoquinone/hydroquinone (Q/H2Q) adlayer on the iodine modified Pd(111) surface.198 At negative potential (0.8 V), all molecules adopt a vertical orientation, and the adlayer has a (2x3 × 4x3)

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16125 or (7 × 2x3) symmetry. In the H2SO4 electrolyte, a more complicated but similar phase transition process is disclosed.134 9. In Situ Monitoring of Surface Chemical Reaction Real time observation of the chemical reaction at the surface and interface is the dream for chemists. Direct examination of a chemical reaction will provide insight into the reaction mechanism and dynamics. On the other hand, the combination of self-assembly and chemical reaction offers a new opportunity to improve the reaction efficiency and obtain sophisticated architecture on the surface. Conjugated polymers have great potential to be used as new generation organic electronic devices including sensors, fieldeffect transistors, light-emitting diodes, solar cell, and so on.204 Electrochemical polymerization is one widely used method to synthesize conjugated polymers on the electrode surface.205 However, the fabrication of well-organized conjugated polymers still is a big challenge. Sakaguchi et al. demonstrate a new electrochemical synthesis process, termed “electrochemical epitaxial polymerization”, to fabricate ordered polythiophene wires on the Au(111) sueface.206 By applying voltage pulses to a thiophene derivative monomer-electrolyte solution that also contains iodine, the high-density arrays of single conjugatedpolymer wires as long as 75 nm are formed on a Au(111) electrode surface. STM is used to observe the growth of polythiophene wires with the potential pulses and investigate the polymerization mechanism. It is revealed that iodine prompts the formation of thiophene trimers in the solution, which adsorb on the iodine covered Au(111) surface and act as the nuclei for the polymer wires. Because of the formation of an ordered iodine adlayer on Au(111), the polymer wires can only propagate along the certain direction on the surface, resulting in the formation of ordered polymer wires. The photoinduced topochemical reaction represents a unique reaction in crystalline conjugated diolefins and diacetylenes.207 The special structural and geometric arrangement of reactive sites of precursors determines the chemical reactivity.208 For example, the prerequisites for photochemical [2+2] reactions in the solid state are that the double bonds of neighboring molecules in the olefin crystal arrange in a parallel fashion and make contact at a distance of 4.2 Å or less.209 In this arrangement, the reactions can proceed in a very efficient way with minimal molecular reorganization. The same reaction can be used in two-dimensions to construct ordered polymer nanostructures at the surface. Previously, the photo- or electricinduced topochemical polymerization has been demonstrated on diacetylene self-assembled monolayer on HOPG.210,211 Yang et al. studied the photopolymerization of 1,4-bis(pyridyl2-vinyl)benzene (P2VB) on Au(111) using ECSTM. First, the P2VB monomer is added into the electrochemical cell. After several minutes, a long-range ordered adlayer is revealed by STM. Individual molecules in a linear configuration with the dimension of 1.52 ( 0.02 nm long and 0.28 ( 0.02 nm wide can be resolved. The molecules assume a flat-lying conformation on the surface. After the STM images of the reagent were obtained, UV light was introduced to irradiate the P2VB adlayer. The steady images were acquired after 55 min of irradiation. A well-defined adlayer is clearly observed after UV irradiation. However, not only the packing arrangement of the adlayer but also the molecular details changed compared to that of the monomer adlayer. Each molecular cluster consists of four spots with different brightnesses. By comparing the STM image and the chemical structure, the cluster is ascribed to a photodimerized product of the P2VB molecule. The four spots correspond to


Figure 19. (a) Large-scale and (b) higher resolution STM images of AOCA adlayer on Au(111) with a bias voltage of -150 mV and a tunneling current of 700 pA. An inserted STM image in (b) shows Au(111) substrate. (c) Proposed structural model for AOCA adlayer on Au(111). (Reprinted with permission from ref 215. Copyright American Chemical Society.)

four aromatic groups of the dimer. The different orientations of dimer moieties with respect to the Au(111) surface result in the contrast difference in the STM images. The size of a dimer is consistent with its chemical structure. On the basis of STM images, the photoinduced [2+2] cycloaddition reaction that occurred in the P2VB adlayer is discussed.212 Cinnamic acid is a typical photoreactive compound. The solidstate dimerization of cinnamic acid is topochemically controlled.213,214 UV light irradiation induced dimeration in a selfassembled monolayer of a cinnamic acid derivative 4-(amyloxy)cinnamic acid (AOCA) on Au(111) has been investigated by using ECSTM.215 The large scale STM image shown in Figure 19a shows that AOCA forms a highly ordered adlayer on the Au(111) surface. The adlayer consists of regular molecular rows along the close-packed direction of the Au(111) lattice and extending over the flat terrace of the substrate surface. From the higher resolution STM image of Figure 19b, the molecular rows in the A direction are composed of several sets of four bright elliptical spots spaced by a row of dark lines. From the chemical structure, it is believed that an elliptical spot corresponds to the cinnamate moiety of an AOCA molecule. The adlayer has a (4 × 11) symmetry. A structural model is proposed in Figure 19c. The dark line in the STM image corresponds to the alkyl chain of AOCA molecules. The adlayer of AOCA is stabilized by intermolecular H-bonds between carboxyl groups. After the AOCA adlayer was imaged, it was irradiated with UV light (365 nm) for ca. 10 min to induce the photochemical reaction. STM images obtained after UV-light irradiation reveal a new packing pattern in the adlayer. The molecular appearance shows a quite different structure as shown in Figure 20a. The adlayer symmetry is changed to a (5 × 8) structure. Furthermore, the structural details are revealed from the higher resolution STM image in Figure 20b. Regular and identical trifolium-shape clusters with sets of three bright spots appear in the adlayer,

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Figure 20. (a) Large scale and (b) higher resolution STM images of the AOCA dimeric adlayer on Au(111) with a bias voltage of -150 mV and a tunneling current of 700 pA. The inserted STM image in (b) is Au(111) substrate. (c) Proposed structural model for the dimeric adlayer. (Reprinted with permission from ref 215. Copyright American Chemical Society.)

indicating the occurrence of photochemical reaction in AOCA adlayer. It is noted that there are pairs of trifolium-shaped clusters in the unit cell. Compared with the chemical structure of a dimer, each “trifolium” consisting of three bright spots is a molecular dimer. Two dimers form a molecular pair as illustrated in Figure 20b. The two dimers take an opposite arrangement in the circle. From the chemical structure and STM results, the product of UV-light induced dimerization should be β-truxinic acid. A structural model for the photochemically resulted dimeric AOCA adlayer is proposed in Figure 20c, in which a (5 × 8) unit cell is outlined. The occurrence of photodimerization is further confirmed by FT-IR study.215 After UV irradiation, the CdC stretching mode at 1625 cm-1 is weakened. At the same time, two new bands at 1170 and 691 cm-1 associated with cyclobutane ring stretching and deformation vibrations are evolved. These results confirm the photodimerization by cyclobutane ring formation. The CdO stretching mode at 1666 cm-1 before irradiation also moved to 1690 cm-1, which is possibly due to the loss of conjugation and the disruption of hydrogen bonds after dimerization. 10. Conclusion and Outlook In this feature article, we have presented the latest progress in adlayer structure and reaction at the solid/liquid interface in aqueous electrolyte solution with ECSTM. With its high resolution, real time, and real space imaging capability, ECSTM plays an important role in clarifying the surface adstructure and physical chemistry process. The adsorption and assembly of organic molecules at the solid/liquid interface is determined by the interplay between molecule/substrate interactions and intermolecular interactions. Understanding the adsorption orientation, structure of organic molecules on surface provides us rich information of the factors governing the formation of adlayer on the surface, which is invaluably important to controllable construct complex archi-

Feature Article tecture of functional molecules on the surface. Innovative assemblies with designable structures have been achieved on the surface either by tuning intermolecular interactions, such as H-bonding, van der Waals interactions, π-π interactions, or electrode/molecule interactions, which can be achieved by modulating the electrode surface charge or electrode surface modification. The active role of potential at the electrochemical solid/liquid interface adds another dimension of control over the orientation of molecules or the organization of the surface assembly on the surface. This perplexing but also fascinating feature of electrochemical adlayer may open new horizons for the development of new functional elements in future sensor and device design. In addition, tailored surface structures on the surface afford the highly efficient, controllable reactions on the surface. By applying proper external stimuli, such as an electrode potential or UV light, reactive precursors can undergo highly efficient chemical reactions on the surface, while keeping the orderliness of the assembly. However, we are still facing challenges related to the construction of molecular nanoarchitectures with specific function and the development of new electronic devices elements, specifically in the context of the rapid development of nanoscience and nanotechnology. As exemplified in this feature article, the atomic level resolution of STM allows us to directly observe the electrode surface structure. New insight into the microscopic picture of the electrode interface information was obtained and used for the establishment of the model of the electric double layer. This ability also helps us to understand the structure-related properties of new electrode materials and will be important in nanoscience and nanotechnology. The “bottom up” supramolecular assembly has seen huge leaps forward in the past decade. Controllable assembly using different noncovalent bonds, such as π-π interactions, H-bonds, and van der Waals interactions, has been obtained with interesting structures. However, for the ultimate device application and to achieve a high integration level, it is desirable to make the assembly process compatible with topdown techniques. In this regard, the use of molecule-substrate interactions and/or surface potential to induce the assembly will be a possible route to achieve the goal. In addition, although noncovalent bonds are more flexible and easy to handle in terms of obtaining a highly ordered assembly with a desirable structure, the covalent bonds certainly have their advantages in terms of high stability and good electron transportation properties.216 Converting the noncovalent bond connected assembly into a covalent bonded rigid structure by external photo or electric field could be a promising solution. Last but not lest, STM is also a powerful tool to manipulate the surface species and create an artificial nanostructure with atomic preciseness.217, By applying external energy input locally on the adlayer, such as mechanical perturbation or electrical field pulse, the nanoscale pattern on the surface can be obtained. For example, Kolb’s group demonstrates the deposition of a Cu nanostructure pattern on an Au electrode surface by ECSTM.218 By combining the ability of STM to pattern a surface and the well-established selfassembly techniques, the possibility to obtain novel nanostructures is endless, which will certainly benefit the molecular machines and electronic devices applications.219,220 Applying the functional single molecules to molecular electronic is a very attractive field.221 Tao’s group has developed a new molecular transistor based on the electrochemical system.222 They use an STM tip and the substrate to connect the molecules and use the reference electrode as a gate electrode. Because of the huge electric field at the electric double layer,

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16127 the current from tip to substrate is modulated by the gate bias. In this way, a single molecular electrochemical transistor is built. The result opens up the new possibility for future molecular electronics developments. For example, it is possible to build up single molecular switch element based on the molecular redox reaction223 or the potential-dependent molecular orientation change. As reported in the present review, ECSTM has imposed a huge impact on almost every field in solid/liquid research with rapid development in the past 20 years. The detailed knowledge of the surface structure provides us a molecular level understanding of the structure at the solid/liquid interface. Complemented by other information from supramolecular chemistry, organometallic chemistry, and UHV surface science, it is reasonable to believe that new heights for the building of complex architectures with specific structures and functions will be achieved in future. Acknowledgment. The financial support from National Natural Science Foundation of China (20575070, 20673121 and 20121301), National Key Project on Basic Research (2006CB806100), and the Chinese Academy of Sciences are gratefully acknowledged. References and Notes (1) Bard, A. J.; Abruna, H. D.; Chidsey, C. E. D.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147-7173. (2) Kolb, D. M. Angew. Chem., Int. Ed. 2001, 40, 1162-1181. (3) Bewick, A.; Kunimatsu, K.; Pons, B. S. Electrochim. Acta. 1980, 25, 465-468. Bewick, A.; Kunimatsu, K.; Pons, B. S. Surf. Sci. 1980, 101, 131-138. (4) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271-340. (5) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861-2880. (6) Tian, Z.-Q.; Ren, B.; Wu, D.-Y. J. Phys. Chem. B 2002, 106, 94639483. (7) Jeanmaire, D. L.; VanDuyne, R. P. J. Electroanal. Chem. 1977, 84, 1-20. (8) Schumacher, R. Angew. Chem. Int. Ed. Engl. 1990, 29, 329-343. (9) (a) Hansen, W. N.; Wang, C. L.; Humpherys, T. W. J. Electroanal. Chem. 1978, 90, 137-141. (b) Hansen, W. N.; Wang, C. L.; Humpherys, T. W. J. Electroanal. Chem. 1978, 93, 87-98. (10) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 325-443. (11) Shannon, C.; Frank, D. G.; Hubbard, A. T. Annu. ReV. Phys. Chem. 1991, 42, 393-431. Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177-184. (12) Binnig, G.; Rohrer, H. HelV. Phys. Acta. 1982, 55, 726-735. Binnig, G.; Rohrer, H. ReV. Mod. Phys. 1987, 59, 615-625. (13) Sonnenfeld, R.; Hansma, P. K. Science. 1986, 232, 211-213. (14) Itaya, K.; Sugawara, S. Chem. Lett. 1987, 16, 1927-1930. (15) Liu, H. Y.; Fan, F. R. F.; Lin, C. W.; Bard, A. J. J. Am. Chem. Soc. 1986, 108, 3838-3839. (16) Itaya, K.; Tomita, E. Surf. Sci. 1988, 201, L507-L512. (17) Kolb, D. M. Surf. Sci. 2002, 500, 722-740. (18) Itaya, K. Prog. Surf. Sci. 1998, 58, 121-247. (19) Yashimoto, S. Bull. Chem. Soc. Jpn. 2006, 79, 1167-1190. (20) Weaver, M. J.; Gao, X. P. Annu. ReV. Phys. Chem. 1993, 44, 459494. (21) Zhang, J.; Chi, Q.; Kuznetsov, A. M.; Hansen, A. G.; Wackerbarth, H.; Christensen, H. E. M.; Andersen, J. E. T.; Ulstrup, J. J. Phys. Chem. B 2002, 106, 1131-1152. (22) Wan, L.-J. Acc. Chem. Res. 2006, 39, 334-342. (23) Gewirth, A. A.; Niece, B. K. Chem. ReV. 1997, 97, 1129-1162. (24) Tao, N.; Li, C.; He, H. J. Electroanal. Chem. 2000, 492, 81-93. (25) (a) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376-379. (b) Sautet, P.; Barbosa, L. A. M. M. J. Am. Chem. Soc. 2001, 123, 6639-6648. (26) Yan, H.-J.; Wang, D.; Han, M.-J.; Wan, L.-J.; Bai, C.-L. Langmuir 2004, 20, 7360-7364. (27) Bonello, J. M.; Lambert, R. M.; Kunzle, N.; Baiker, A. J. Am. Chem. Soc. 2000, 122, 9864-9865. Bonalumi, N.; Burgi, T.; Baiker, A. J. Am. Chem. Soc. 2003, 125, 13342-13343. (28) (a) Xu, Q.-M.; Wang, D.; Wan, L.-J.; Bai, C.-L.; Wang, Y. J. Am. Chem. Soc. 2002, 124, 14300-14301. (b) Xu, Q.-M.; Wang, D.; Han, M.J.; Wan, L.-J.; Bai, C.-L. Langmuir 2004, 20, 3006-3010. (29) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1999, 254, 1312-1319.

16128 J. Phys. Chem. C, Vol. 111, No. 44, 2007 (30) Zhu, L.; Claude-Montigny, B.; Gattrell, M. Appl. Surf. Sci. 2005, 252, 1833-1845. (31) Abelev, E.; Sezin, N.; Ein-Eli, Y. ReV. Sci. Instrum. 2005, 76, 106105. (32) (a) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205-209. (b) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211-216. (c) Clavilier, J.; Rodes, A.; El Achi, K.; Zamakhchari, M. A. J. Chim. Phys. 1991, 88, 1291-1337. (33) Hamelin, A.; Doubova, L.; Wagner, D.; Schirmer, H. J. Electroanal. Chem. 1987, 220, 155-160. (34) Wan, L.-J.; Hara, M.; Inukai, J.; Itaya, K. J. Phys. Chem. B 1999, 103, 6978-6983. (35) Wan, L.-J.; Suzuki, T. Sashikata, K. Okada, J.; Inukai, J.; Itaya, K. J. Electroanal. Chem. 2000, 484, 189-193. (36) Wan, L. J.; Yau, S. L.; Swain, G. M.; Itaya, K. J. Electroanal. Chem. 1995, 381, 105-111. (37) Wan, L.-J.; Itaya, K. Langmuir 1997, 13, 7173-7179. (38) Suzuki, T.; Yamada, T.; Itaya, K. J. Phys. Chem. 1996, 100, 89548961. (39) Kong, D.-S.; Chen, S.-H.; Wan, L.-J.; Han, M.-J. Langmuir 2003, 19, 1954-1957. (40) Weiss, P. S.; Eigler, D. M. Phys. ReV. Lett. 1993, 71, 3139-3142. (41) (a) Sautet, P.; Bocquet, M. L. Surf. Sci. 1994, 304, L445-L450. (b) Sautet, P.; Bocquet, M. L. Isr. J. Chem. 1996, 36, 63-72. (42) Sautet, P. Chem. ReV. 1997, 97, 1097-1116. (43) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109-173. (44) Schneider, K. S.; Nicholson, K. T.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2002, 18, 8116-8122. (45) Magnussen, O. M.; Zitzler, L.; Gleich, B.; Vogt, M. R.; Behm, R. J. Electrochim. Acta. 2001, 46, 3725-3733. (46) Magnussen, O. M.; Vogt, M. R. Phys. ReV. Lett. 2000, 85, 357360. (47) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543-561. Stickney, J. L. Electroanal. Chem. 1999, 21, 75-209. (48) (a) Yau, S.-L.; Kaji, K.; Itaya, K. Appl. Phys. Lett. 1995, 66, 766768. (b) Yau, S. L.; Fan, F. R.; Bard, A. J. J. Electrochem. Soc. 1992, 139, 2825-2829. (49) (a) Allongue, P.; Costa-Kieling, V.; Gerischer, H. J. Electrochem. Soc. 1993, 140, 1009-1018. (b) Allongue, P.; Costa-Kieling, V.; Gerischer, H. J. Electrochem. Soc. 1993, 140, 1018-1026. (50) (a) Watanabe, M.; Zhu, Y.; Uchida, H. J. Phys. Chem. B 2000, 104, 1762-1768. (b) Igarashi, H.; Fujino, T.; Zhu, Y.; Uchida, H.; Watanabe, M. Phys. Chem. Chem. Phys. 2001, 3, 306-314. (c) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750-3756. (51) Wan, L.-J. Moriyama, T.; Ito, M.; Uchida, H.; Watanabe, M. Chem. Commun. 2002, 58-59. (52) Magnussen, O. M. Chem. ReV. 2002, 102, 679-726. (53) Markovic, N.; Gasteiger, H. A.; Ross, P. N. J. Phys. Chem. 1995, 99, 3411-3415. (54) Schimpf, J. A.; McBride, J. R.; Soriaga, M. P. J. Phys. Chem. 1993, 97, 10518-10520. (55) (a) Shue, C.-H.; Yau, S.-L. J. Phys. Chem. B 2001, 105, 54895496. (b) Ou Yang, L.-Y.; Bensliman, F.; Shue, C.-H.; Yang, Y.-C.; Zang, Z.-H.; Wang, L.; Yau, S.-L.; Yoshimoto, S.; Itaya, K. J. Phys. Chem. B 2005, 109, 14917-14924. (56) Hubbard, A. T. Chem. ReV. 1988, 88, 633-656. (57) Soriaga, M. P. Chem. ReV. 1990, 90, 771-793. (58) Gao, X.; Weaver, M. J. J. Am. Chem. Soc. 1992, 114, 8544-8551. (59) (a) Schardt, B. C.; Yau, S.-L.; Rinaldi, F. Science 1989, 243, 10501053. (b) Yau, S.-L.; Vitus, C. M.; Schardt, B. C. J. Am. Chem. Soc. 1990, 112, 3677-3679. (60) Foresti, M. L.; Aloisi, G.; Innocenti, M.; Kobayashi, H.; Guidelli, R. Surf. Sci. 1995, 335, 241-251. (61) (a) Inukai, J.; Osawa, Y.; Itaya, K. J. Phys. Chem. B 1998, 102, 10034-10040. (b) Suggs, D. W.; Bard, A. J. J. Phys. Chem. B 1994, 99, 8349-8355. (c) Suggs, D. W.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 10725-10733. (62) Sashikata, K.; Matsui, Y.; Itaya, K.; Soriaga, M. P. J. Phys. Chem. 1996, 100, 20027-20034. (63) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J. Electroanal. Chem. Interfac. Electrochem. 1987, 222, 305-322. (64) Schardt, B. C.; Yau, S. L.; Rinaldi, F. Science. 1989, 243, 10501053. (65) Inukai, J.; Osawa, Y.; Wakisaka, M.; Sashikata, K.; Kim, Y. G.; Itaya, K. J. Phys. Chem. B 1998, 102, 3498-3505. (66) (a) Batina, N.; Yamada, T.; Itaya, K. Langmuir 1995, 11, 45684576. (b) Yamada, T.; Batina, N.; Itaya, K. J. Phys. Chem. 1995, 99, 88178823. (67) Yamada, T.; Ogaki, K.; Okubo, S.; Itaya, K. Surf. Sci. 1996, 369, 321-335.

Wang and Wan (68) (a) Faguy, P. W.; Marinkovic, N. S.; Adzic, R. R. Langmuir 1996, 12, 243-247. (b) Shingaya, Y.; Ito, M. J. Electroanal. Chem. 1999, 467, 299-306. (69) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951-959. (70) Shi, Z.; Lipkowski, J.; Mirwald, S.; Pettinger, B. J. Electroanal. Chem. 1995, 396, 115-124. (71) Magnussen, O. M.; Hagebbock, J.; Hotlos, J.; Behm, R. J. Faraday Discuss. Chem. Soc. 1992, 94, 329-338. (72) Edens, G. J.; Gao, X. P.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357-366. (73) Wan, L. J.; Yau, S. L.; Itaya, K. J. Phys. Chem. 1995, 99, 95079513. (74) Zelenay, P.; Wieckowski, A. J. Electrochem. Soc. 1992, 139, 25522558. (75) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211-385. (76) Wilms, M.; Broekmann, P.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1998, 416, 121-140. (77) Wan, L. J.; Suzuki, T.; Sashikata, K.; Okada, J.; Inukai, J.; Itaya, K. J. Electroanal. Chem. 2000, 484, 189-193. (78) (a) Funtikov, A. M.; Stimming, U.; Vogel, R. J. Electroanal. Chem. 1997, 428, 147-153. (b) Funtikov, A. M.; Linke, U.; Stimming, U.; Vogel, R. Surf. Sci. 1995, 324, L343-L348. (79) Campbell, C. T.; Koel, B. E. Surf. Sci. 1987, 183, 100. (80) de Tacconi, N. R.; Rajeshwar, K.; Lezna, R. O. J. Phys. Chem. 1996, 100, 18234-18239. (81) Wang, D.; Xu, Q.-M.; Wan, L.-J.; Wang, C.; Bai C.-L. Surf. Sci. 2002, 499, L159-L163. (82) Ruan, L.; Stensgaard, I.; Besenbacher F.; Lægsgaard, E. Ultramicroscopy 1992, 42-44, 498. (83) (a) Sugimasa, M.; Inukai, J.; Itaya, K. J. Electrochem. Soc. 2003, 150, E110-E116. (b) Sugimasa, M.; Inukai, J.; Itaya, K. J. Electroanal. Chem. 2003, 554, 285-291. (84) Herrero, E.; Buller, L. J.; Abruna, H. D. Chem. ReV. 2001, 101, 1897-1930. (85) Obretenov, W.; Schmidt, U.; Lorenz, W. J.; Staikov, G.; Budevski, E.; Carnal, D.; Muller, U.; Siegenthaler, H.; Schmidt, E. J. Electrochem. Soc. 1993, 140, 692-703. (86) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M. Phys. ReV. Lett. 1990, 64, 2929-2932. (87) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183-186. (88) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Yee, D.; Sorensen, L. B. Phys. ReV. Lett. 1995, 75, 4472-4475. (89) Batina, N.; Will, T.; Kolb, D. M. Faraday Discuss. 1992, 94, 93106. (90) Hotlos, J.; Magnussen, O. M.; Behm, R. J. Surf. Sci. 1995, 335, 129-144. (91) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275-291. (92) Wu, S.; Lipkowski, J.; Tyliszczak, T.; Hitchcock, A. P. Prog. Surf. Sci. 1995, 50, 227-236. (93) (a) Hommrich, J.; Humann, S.; Wandelt, K. Faraday Discuss. 2002, 121, 129-138. (b) Arenz, M.; Stamenkovic, V.; Schmidt, T. J.; Wandelt, K.; Ross, P. N.; Markovic, N. M. Surf. Sci. 2003, 523, 199-209. (94) Oyamatsu, D.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 1999, 473, 59-67. (95) Danilov, A. I.; Molodkina, E. B.; Rudnev, A. V.; Polukarov, Y. M.; Feliu, J. M. Electrochim. Acta. 2005, 50, 5032-5043. (96) Kuzume, A.; Herrero, E.; Feliu, J. M.; Nichols, R. J.; Schiffrin, D. J. J. Electroanal. Chem. 2004, 570, 157-161. (97) (a) Kukta, R. V.; Vasiljevic, N.; Dimitrov, N.; Sieradzki, K. Phys. ReV. Lett. 2005, 95, 186103. (b) Vasiljevic, N.; Dimitrov, N.; Sieradzki, K. J. Electroanal. Chem. 2006, 595, 60-70. (98) Randler, R.; Dietterle, M.; Kolb, D. M. Z. Phys. Chem. 1999, 208, 43-56. (99) Dietterle, M.; Will, T.; Kolb, D. M. Surf. Sci. 1998, 396, 189197. (100) Randler, R. J.; Kolb, D. M.; Ocko, B. M.; Robinson, I. K. Surf. Sci. 2000, 447, 187-200. (101) Ocko, B. M.; Robinson, I. K.; Weinert, M.; Randler, R. J.; Kolb, D. M. Phys. ReV. Lett. 1999, 83, 780-783. (102) Sieradzki, K.; Rankovic, S. R.; Dimitrov, N. Science 1999, 284, 138-141. (103) Rosenfel, G.; Servaty, R.; Teichert, C.; Poelsema, B.; Comsa, G. Phys. ReV. Lett. 1993, 71, 895-898. (104) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173-L179. (105) Vasilic, R.; Dimitrov, N. Electrochem. Solid-State Lett. 2005, 8, C173-C176. (106) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Anal. Chem. 2001, 73, 5953-5960.

Feature Article (107) Kim, Y. G.; Kim, J. Y.; Vairavapandian, D.; Stickney, J. L. J. Phys. Chem. B 2006, 110, 17998-18006. (108) (a) Lay, M. D.; Stickney, J. L. J. Electrochem. Soc. 2004, 151, C431-C435. (b) Varazo, K.; Lay, M. D.; Sorenson, T. A.; Stickney, J. L. J. Electroanal. Chem. 2002, 522, 104-114. (109) Lister, T. E.; Stickney, J. L. Appl. Surf. Sci. 1996, 107, 153-160. (110) Villegas, I.; Stickney, J. L. J. Vac. Sci. Technol. A 1992, 10, 30323038. (111) Barth, J. V.; Costantini, G.; Kern K. Nature 2005, 437, 671679. (112) (a) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520-531. (b) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290-4302. (113) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491501. (114) (a) Qiu, X.; Wang, C.; Zeng, Q.; Xu, B.; Yin, S.; Wang, H.; Xu, S.; Bai, C. J. Am. Chem. Soc. 2000, 122, 5550-5556. (b) Lei, S. B.; Wang, C.; Yin, S. X.; Wang, H. N.; Xi, F.; Liu, H. W.; Xu, B.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2001, 105, 10838-10841. (115) (a) Clair, S.; Pons, S.; Seitsonen, A. P.; Brune, H.; Kern, K.; Barth, J. V. J. Phys. Chem. B 2004, 108, 14585-14590. (b) Barth, J. V.; Weckesser, J.; Trimarchi, G.; Vladimirova, M.; De Vita, A.; Cai, C.; Brune, H.; Gunter, P.; Kern, K. J. Am. Chem. Soc. 2002, 124, 7991-8000. (116) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795-7803. (117) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1997, 101, 3547-3553. (118) Wang, D.; Wan, L.-J.; Xu, Q.-M.; Wang, C.; Bai, C.-L. Surf. Sci. 2001, 478, L320-L326. (119) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 23372340. (120) Wan, L.-J.; Shundo, S.; Inukai, J.; Itaya, K. Langmuir 2000, 16, 2164-2168. (121) Gao, X. P.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, 96, 4156-4159. (122) (a) Vericat, C.; Andreasen, G.; Vela, M. E.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 302-307. (b) Martin, H.; Vericat, C.; Andreasen, G.; Creus, A. H.; Vela, M. E.; Salvarezza, R. C. Langmuir 2001, 17, 23342339. (123) (a) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245-250. (b) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607-1615. (124) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185-7190. (125) Sashikata, K.; Sugata, T.; Sugimasa, M.; Itaya, K. Langmuir 1998, 14, 2896-2902. (126) Itaya, K.; Batina, N.; Kunitake, M.; Ogaki, K.; Kim, Y.-G.; Wan, L.-J.; Yamada, T. In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., Soriaga, M. P., Uosaki, K., Wieckowski, A., Eds.; ACS Symposium Series 656; American Chemical Society: Washington, DC, 1997; pp 171188. (127) Wang, D.; Wan, L.-J.; Wang, C.; Bai, C.-L. J. Phys. Chem. B 2002, 106, 4223-4226. (128) Su, G.-J.; Gan, L.-H.; Yang, Z.-Y.; Pan, G.-B.; Wan, L.-J.; Wang, C.-R. J. Phys. Chem. B 2006, 110, 5559-5562. (129) Safarowsky, C.; Merz, L.; Rang, A.; Broekmann, P.; Hermann, B. A.; Schalley, C. A. Angew. Chem., Int. Ed. 2004, 43, 1291-1294. (130) Uemura, S.; Sakata, M.; Hirayama, C.; Kunitake, M. Langmuir 2004, 20, 9198-9201. (131) (a) Uemura, S.; Ohira, A.; Ishizaki, T.; Sakata, M.; Taniguchi, I.; Kunitake, M.; Hirayama, C. Chem. Lett. 1999, 28, 279-280. (b) Uemura, S.; Ohira, A.; Sakata, M.; Taniguchi, I.; Kunitake, M.; Hirayama, C. Langmuir 2001, 17, 5-7. (132) Ishikawa, Y.; Ohira, A.; Sakata, M.; Hirayama, C.; Kunitake, M. Chem. Commun. 2002, 2652-2653. (133) Yan, H.-J.; Lu, J.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. B 2004, 108, 11251-11255. (134) Li, Z.; Han, B.; Wan, L. J.; Wandlowski, Th. Langmuir 2005, 21, 6915-6928. (135) (a) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. M. Langmuir 2004, 20, 9403-9407. (b) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25-31. (136) Lu, J.; Zeng, Q.-D.; Wang, C.; Zheng, Q.-Y.; Wan L.-J.; Bai, C. L. J. Mater. Chem. 2002, 12, 2856-2858. (137) Dmitriev, A.; Lin, N.; Weckesser, J.; Barth, J. V.; Kern, K. J. Phys. Chem. B 2002, 106, 6907-6912. (138) Klymchenko, A. S.; Furukawa, S.; Mullen, K.; Van der Auweraer, M.; De Feyter, S. Nano Lett. 2007, 7, 791-795. (139) Xie, Z. X.; Xu, X.; Tang, J.; Mao, B. W. J. Phys. Chem. B 2000, 104, 11719-11722.

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16129 (140) (a) Yamada, R.; Uosaki, K. J. Phys. Chem. B 2000, 104, 60216027. (b) Uosaki, K.; Yamada, R. J. Am. Chem. Soc. 1999, 121, 40904091. (141) He, Y. Ye, T. Borguet E. J. Phys. Chem. B 2002, 106, 1126411271. (142) Dinolfo, P. H.; Hupp, J. T. Chem. Mater. 2001, 13, 3113-3125. (143) Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. ReV. 1999, 99, 1863-1933. (144) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022-2043. (145) Fujita, M. Chem. Soc. ReV. 1998, 27, 417-425. (146) Xu, Q. M.; Zhang, B.; Wan, L. J.; Wang, C.; Bai, C. L.; Zhu, D. B. Surf. Sci. 2002, 517, 52-58. (147) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 41, 3681-3683. (148) Figgemeier, E.; Merz, L.; Hermann, B. A.; Zimmermann, Y. C.; Housecroft, C. E.; Guntherodt, H. J.; Constable, E. C. J. Phys. Chem. B 2003, 107, 1157-1162. (149) Ma, H.; Ou Yang, L.-Y.; Pan, N.; Yau, S.-L.; Jiang, J.; Itaya, K. Langmuir 2006, 22, 2105-2111. (150) Semenov, A.; Spatz, J. P.; Moller, M.; Lehn, J.-M.; Sell, B.; Schubert, D.; Weidl, C. H.; Schubert, U. S. Angew. Chem., Int. Ed. 1999, 38, 2547-2550. (151) Ziener, U.; Lehn, J. M.; Mourran, A.; Moller, M. Chem. Eur. J. 2002, 8, 951-957. (152) Yuan, Q.-H.; Wan, L.-J.; Jude, H.; Stang, P. J. J. Am. Chem. Soc. 2005, 127, 16279-16286. (153) Gong, J. R.; Wan, L. J.; Yuan, Q. H.; Bai, C. L.; Jude, H.; Stang, P. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 971-974. (154) Kuehl, C. J.; Songping, D. H.; Stang, P. J. J. Am. Chem. Soc. 2001, 123, 9634-9641. (155) (a) Gong, J.-R.; Yan, H.-J.; Yuan, Q.-H.; Xu, L.-P.; Bo, Z.-S.; Wan, L.-J. J. Am. Chem. Soc. 2006, 128, 12384-12385. (b) Lu, J.; Lei, S.-B.; Zeng, Q.-D.; Kang, S.-Z.; Wang, C.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. B 2004, 108, 5161-5165. (c) Lei, S. B.; Yin, S. X.; Wang, C.; Wan, L. J.; Bai, C. L. Chem. Mater. 2002, 14, 2837-2838. (156) De Feyter, S.; Miura, A.; Yao, S.; Chen, Z.; Wurthner, F.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.; De Schryver, F. C. Nano Lett. 2005, 5, 77-81. (157) Gesquiere, A.; De Feyter, S.; De Schryve, F. C.; Schoonbeek, F.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Nano Lett. 2001, 1, 201-206. (158) (a) Messina, P.; Dmitriev, A.; Lin, N.; Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 14000-14001. (b) Dmitriev, A.; Spillmann, H.; Lingenfelder, M.; Lin, N.; Barth, J. V.; Kern, K. Langmuir 2004, 20, 4799-4801. (159) (a) Curl, R. F. Angew. Chem., Int. Ed. Engl. 1997, 36, 15661576. (b) Kroto, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 1578-1593. (160) Sherigara, B. S.; Kutner, W.; D’Souza, F. Electroanalysis. 2003, 15, 753-772. (161) Marchenko, A.; Cousty, J. Surf. Sci. 2002, 513, 233-237. (162) Yoshimoto, S.; Narita, R.; Tsutsumi, E.; Matsumoto, M.; Itaya, K.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu, K. Langmuir 2002, 18, 8518-8522. (163) Suzuki, T.; Nakashima, K.; Shinkai, S. Chem. Lett. 1994, 23, 699702. (164) Sakai, T.; Ohira, A.; Sakata, M.; Hirayama, C.; M. Kunitake, Chem. Lett. 2001, 30, 782-783. (165) Schonherr, H.; Vancso, G. J.; Huisman, B.-H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1999, 15, 5541-5546. (166) Yoshimoto, S.; Abe, M.; Itaya, K.; Narumi, F.; Sashikata, K.; Nishiyama, K.; Taniguchi, I. Langmuir 2003, 19, 8130-8133. (167) (a) Pan, G. B.; Bu, J. H.; Wang, D.; Liu, J. M.; Wan, L. J.; Zheng, Q. Y.; Bai, C. L. J. Phys. Chem. B 2003, 107, 13111-13116. (b) Pan, G. B.; Wan, L. J.; Zheng, Q. Y.; Bai, C. L.; Itaya, K. Chem. Phys. Lett. 2002, 359, 83-88. (c) Pan, G. B.; Wan, L. J.; Zheng, Q. Y.; Bai, C. L. Chem. Phys. Lett. 2003, 367, 711-716. (168) Pan, G. B.; Liu, J. M.; Zhang, H. M.; Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2003, 42, 2747-2751. (169) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40-48. (170) Da Ros, T.; Prato, M.; Guldi, D. M.; Ruzzi, M.; Pasimeni, L. Chem. Eur. J. 2001, 7, 816-827. (171) Ishii, T.; Aizawa, N.; Kanehama, R.; Yamashita, M.; Sugiura, K.I.; Miyasaka, H.; Coord. Chem. ReV. 2002, 226, 113-124. (172) Meijer, M. D.; van Klink, G. P. M.; van Koten, G. Coord. Chem. ReV. 2002, 230, 141-163. (173) Boyd, P. D. W.; Reed, C. A. Acc. Chem. Res. 2005, 38, 235242. (174) Sto¨hr, M.; Wagner, T.; Gabriel, M.; Weyers, B.; Mo¨ller, R. AdV. Funct. Mater. 2001, 11, 175-178. (175) Wild, M.; Berner, S.; Suzuki, H.; Yanagi, H.; Schlettwein, D.; Ivan, S.; Baratoff, A.; Guentherodt, H.-J.; Jung, T. A. Chem. Phys. Chem. 2002, 3, 881-885.

16130 J. Phys. Chem. C, Vol. 111, No. 44, 2007 (176) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. Angew. Chem., Int. Ed. 2004, 43, 30443047. (177) (a) Yoshimoto, S.; Inukai, J.; Tada, A.; Abe, T.; Morimoto, T.; Osuka, A.; Furuta, H.; Itaya, K. J. Phys. Chem. B 2004, 108, 1948-1954. (b) Yoshimoto, S.; Tada, A.; Itaya, K. J. Phys. Chem. B 2004, 108, 51715174. (178) Yoshimoto, S.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. J. Phys. Chem. B 2005, 109, 8547-8550. (179) Yoshimoto, S.; Saito, A.; Tsutsumi, E.; D’Souza, F.; Ito, O.; Itaya, K. Langmuir 2004, 20, 11046-11052. (180) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 1537-1554. (181) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619-621. (182) (a) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P., Jr. J. Am. Chem. Soc. 2002, 124, 2126-2127. (b) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073-4080. (183) (a) Suto, K.; Yoshimoto, S.; Itaya, K. Langmuir 2006, 22, 1076610776. (b) Suto, K.; Yoshimoto, S.; Itaya, K. J. Am. Chem. Soc. 2003, 125, 14976-14977. (c) Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540-8545. (184) de Levie, R. Chem. ReV. 1988, 88, 599-609. (185) He, Y.; Ye, T.; Borguet, E. J. Am. Chem. Soc. 2002, 124, 1196411970. (186) Ye, T.; He, Y.; Borguet, E. J. Phys. Chem. B 2006, 110, 61416147. (187) Wandlowski, Th. J. Electroanal. Chem. 1995, 395, 83-89. (188) (a) Wandlowski, Th.; Holzle, M. H. Langmuir 1996, 12, 66046615. (b) Holzle, M. H.; Wandlowski, Th.; Kolb, D. M. J. Electroanal. Chem. 1995, 394, 271-275. (c) Wandlowski, Th.; Lampner, D.; Lindsay, S. M. J. Electroanal. Chem. 1996, 404, 215-226. (d) Holzle, M.; Wandloski, T.; Kolb, D. M. Surf. Sci. 1995, 335, 281-290. (189) (a) Yang, D.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Mirwald, S. J. Phys. Chem. 1994, 98, 7083-7089. (b) Hoon-Koshla, M.; Fawcett, W. R.; Goddard, J. D.; Tian, W.-Q.; Lipkowski, J. Langmuir 2000, 16, 2356-2362. (190) Hiroyuki, N.; Minoha, T.; Wan, L.-J.; Osawa, M. J. Electroanal. Chem. 2000, 481, 62-68. (191) (a) Cunha, F.; Tao, N. J. Phys. ReV. Lett. 1995, 75, 2376-2379. (b) Cunha, F.; Tao, N. J.; Wang, X. W.; Jin, Q.; Duong, B.; D’Agnese, J. Langmuir 1996, 12, 6410-6418. (192) Wan, L. J.; Noda, H.; Wang, C.; Bai, C. L.; Osawa, M. Chem. Phys. Chem. 2001, 2, 617-619. (193) Dretschkow, T.; Lampner, D.; Wandlowski, T. J. Electroanal. Chem. 1998, 458, 121-138. (194) Diao, Y.-X.; Han, M.-J.; Wan, L.-J.; Itaya, K.; Uchida, T.; Miyake, H.; Yamakata, A.; Osawa, M. Langmuir 2006, 22, 3640-3646. (195) Lu, T.; Cotton, T. M. Langmuir 1989, 5, 406-414. (196) Ito, M.; Sasaki, H.; Takahashi, M. J. Phys. Chem. 1987, 91, 39323934. (197) Osawa, M.; Yoshii, K. Appl. Spectrosc. 1997, 51, 512-518.

Wang and Wan (198) Kim, Y.-G.; Soriaga, M. P. J. Colloid Interface Sci. 2001, 236, 197-199. (199) Mayer, D.; Dretschkow, Th.; Ataka, K.; Wandlowski, Th. J. Electroanal. Chem. 2002, 524-525, 20-35. (200) Wandlowski, Th.; Ataka, K.; Mayer, D. Langmuir 2002, 18, 4331-4341. (201) Cai, W.-B.; Wan, L.-J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992-6998. (202) Su, G.-J.; Zhang, H.-M.; Wan, L.-J.; Bai, C.-L.; Wandlowski, T. J. Phys. Chem. B 2004, 108, 1931-1937. (203) Zelenay, P.; Waszczuk, P.; Dobrowolska, K.; Sobkowski, J. Electrochim. Acta. 1994, 39, 655. (204) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537-2574. (205) Roncali, J. Chem. ReV. 1992, 92, 711-738. Roncali, J. Chem. ReV. 1997, 97, 173-206. (206) Sakaguchi, H.; Matsumura, H.; Gong, H. Nat. Mater. 2004, 3, 551-557. (207) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433481. (208) Saragai, S.; Tashiro, K.; Nakamoto, S.; Matsumoto, A.; Tsubouchi, T. J. Phys. Chem. B 2001, 105, 4155-4165. (209) Zimmerman, H. E.; Nesterov, E. E. Acc. Chem. Res. 2002, 35, 77-85. (210) Okawa, Y.; Aono, M. Nature 2001, 409, 683-684. (211) Qiao, Y. H.; Zeng, Q. D.; Tan, Z. Y.; Xu, S. D.; Wang, D.; Wang, C.; Wan, L. J.; Bai, C. L. J. Vac. Sci. Technol. B 2002, 20, 2466-2469. (212) Yang, G.-Z.; Wan, L.-J.; Zeng, Q.-D.; Bai, C.-L. J. Phys. Chem. B 2003, 107, 5116-5119. (213) Dilling, W. L. Chem. ReV. 1983, 83, 1-47. (214) Hasegawa, M. Chem. ReV. 1983, 83, 507-518. (215) Xu, L.-P.; Yan, C.-J.; Wan, L.-J.; Jiang, S.-G.; Liu, M.-H. J. Phys. Chem. B 2005, 109, 14773-14778. (216) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571-574. (217) Nyffenegger, R. M.; Penner, R. M. Chem. ReV. 1997, 97, 11951230. (218) (a) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 10971099. (b) Kolb, D. M.; Engelmann, G. E.; Ziegler, J. C. Angew. Chem., Int. Ed. 2000, 39, 1123-1125. (219) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457466. (220) Kramer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367-4418. (221) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173-181. (222) (a) Xu, B. Q.; Xiao, X. Y.; Yang, X.; Zang, L.; Tao, N. J. J. Am. Chem. Soc. 2005, 127, 2386-2387. (b) Xiao, X.; Nagahara, L. A.; Rawlett, A. M.; Tao, N. J. J. Am. Chem. Soc. 2005, 127, 9235-9240. (223) Li, Z.; Han, B.; Meszaros, G.; Pobelov, I.; Wandlowski, Th.; Baszczyk, A.; Mayor, M. Faraday Discuss. 2006, 131, 121-143.

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